[Poleshi]

Sup guys   
Well, me and some of my friends started to play Shadowrun 5th couple of weeks ago,and we love it.We have already stolen some corps devices from facilities and important files.Poor corps...
Actually,this is the thing that i dont understand.
The Megacorps are powerfull.They have billions of nuyen.They have Johnsons and connections with runners.
So, when they can start to hunt runners?
If someone stole prototype of new gun from your facility,is it enough for you to open the hunt season?If couple of your guards was killed during the extraction is it enough?What exactly they know when its time to kill runners?
This is very important theme for me,because i dont want to just throw elite corporate squad to my players and say-"You are dead.Not big surprice."
Is it any books or pieces of lore or rules for this moment?i dont  wanna to think that corps are so powerless that they cant find some lucky runners.But i dont know what the actually balance here...
Help me please guys.
P.S. I am sorry for my awfull english,i am not a native speaker 

[PiXeL01]

It all depends on the bottom line. If the runners have hurt the corp really bad and it would make sense economically to go after that runners then they'll come.
It also depends on how much they brag about hitting that corp.

Runners these days are part of business and are even on the budget of all bigger corps. It's all a question of cost-benefit. They might make an attempt to get their merchandise back.
Killing runners for revenge is bad business

[BetaCAV]

How subtle were they about it?
If most of the corp they stole it from isn't aware of it yet, the focus may just be on getting it back, as quickly and quietly as possible.
If a competitor has discovered the theft, they may send a team to acquire it, keep it just long enough to scan it for later analysis, and then hire some runners to give it back (read as "get caught with it"), before the whole thing gets too public.
Really, the prototype's  value is mostly in the design, rather than the prototype itself, and then only to the degree that the design is proprietary/sekrit.
A collector may be willing to pay a bundle for it, just to hide it away, and know they've got something no one else does (or likely, ever will). But since they can never openly sell it, or even show it off much, it's value is reduced even to collectors.
If anyone was really on the ball where it was stolen from, it may have an embedded RFID that will only respond to a coded signal, allowing localized tracking, and using that to see where it ends up... information which may prove to be more valuable than the prototype (which may be flawed anyway).

[CitizenJoe]

An important thing to remember is that it is more important to the corporation that the stolen files/tech/prototype doesn't fall into rival hands that it is for them to get it back.  Generally speaking, they can reproduce the results, make a new prototype, etc.   Once the opposition gets the stolen data, they can rush it to market and beat the original corp to the punch.  So, once the data is destroyed or rendered valueless, chasing runners is just a waste of money.  Likewise, once the data is delivered, chasing runners is just an expensive revenge fantasy.

[TheWayfinder]

As others have said, basically, it depends on a lot of things, but here's something to consider, at least from my vantage as a GM.

A Corporation, be they mega or minor, or even just a small business, is comprised of people, and a lot of them want to get promoted.  Many of them may be wage slaves, some may have some management or junior executive position, and a few may have some more senior executive status, but among the latter two, upward mobility is always on their minds.  If a manager or junior executive finds out that something, however trivial, has been stolen from his company, he may weigh the options of going after the runners who stole it and recover what was taken.  This is especially important because letting it go might actually threaten his chances of promotion, or even get him fired.  The corporate world is dog-eat-dog, and it gets very political. 

So, this widens your options a bit.  Say the PCs stole something that's valuable, but rather minor in the greater scheme of things.  The person overseeing the area of operations where that item was stolen could now find himself in hot water, or may see this as a golden opportunity to show that he's pro-active, and will use whatever resources he has available to hunt down the PCs and recover what was stolen. 

It doesn't have to be a big operation to hunt down the PCs, even if it's a megacorp behind it.  What this does is make the megacorps a bit less monolithic in tone as far as the players are concerned.  They're less faceless.  You now have an opportunity to make a villain, even if it's a relatively minor one.  A manager or junior exec could be a major thorn in the PCs sides. 

For example:  Suppose one of your decker PCs decides to go solo and rob a company of a little paydata.  Though she was successful, the jr. exec overlooking the area figures out what happened and begins privately marshalling his assets in order to recover the data.  So, he gets in touch with some company men, who know the seedier aspects of society, and they, in turn, get in touch with a fixer, and pay him to find out who infiltrated X Corp Facility at 0039 hrs Monday morning, at X location.  To do that, the fixer hires a decker, maybe a detective, to track the thief down.  Then, once located, sooner or later, the exec then hires a team to capture her.  And now you got an adventure in your hands.

[Beta]

An important factor to remember is that the 'runners are not generally advertising their identity.  Most will go in with their face obscured or disguised in some way, be at pains not to leave tissue samples (aka blood splatter) around, casters will try to clean up their spell signatures, they'll try not to come close to the target site in vehicles that they own, they'll use approach and departure routes that get away from monitored areas at times so that a good decker can't trace their signature through the city to their homes, etc.  And on top of that, most will change their fake identities periodically (not to mention having back ups, so that anything related to a run can be done while using a different ID than what they normally live with).

All of which means that corps can expect to have to put pretty serious resources into:
-  identifying the runners who hit them,
- tracking them down
- keeping track of them long enough to organize the hit.

Given that runners are just hired guns for someone else, and how hard it can be to punish them, usually corps won't pursue revenge heavily.  But if the runners make it easy to find them, or they especially infuriate some branch of the company (say the security organization by going to pains to kill beyond hope of revival a lot of security guards), then things can change.  Runners should always be ready to grab one bag and make a run for it, leaving everything else behind (including the blown identity).

Of course, the reaction of corps could equally well be "if these guys are so good, we should be hiring them to hit our rivals...."

[shreck]

hope this makes sence :
first off the runners did not steal the prototype or the files .
someone els did and used the runners as his tools to do it .
as long as the runners are in possesion of the "item" thy will be hunted down but once thy have dropt it off thy are in the clear .
that is imo the mindset of the corp . ( heck your next johnson might be from that corp )

of cource like  TheWayfinder said you olso got the people that work there to consider :
did you kill a guard ?
oops your bad it turnd out the be a lover of a big shot and now he is out for blood.
the guard was on the take/member of a gang and now thy are looking for you becous you cost them a resource.
the head researcher/securitie was demoted becous of the theft and now has a bone to pick whith you .
you might not have been the only team after that device and now you are responssible for another team losing reputation .

[Sternenwind]

There can be other factors leading to a manhunt.

Security efficiency – you ruined some ones and with that you endangered his job, pension or bonus.
Image / reputation – you ruined some ones and they are forced or encouraged to make an example out of you.
FuckUp – Whatever you did, knowing or unknowing you uncover someone elses fuckup and he may be forced to deal with you to cover his tracks. Like preventing a investigation that would uncover something bad.
Money left in the budget – this may sound strange, but there is money left in the budget for this quarter, they just may spend it (throw it away) for a manhunt to gain some rep before the end of the quarter.*
Money left in the budget - this may sound strange, but there is money left in the budget for this quarter and they may spend it to prevent budget shorting’s or to get an budget increase.*

*This stuff is really happening in governments and firms.

[friczvonbrock]

I think that a Mega will see that just like business and will be in anger with the Mr. Johnson behind the run, and not the runners.
Maybe they'll use the runners like example to other runners don't try to do the same thing, but I dunno, I think that wouldn't be a good idea, the Mega has a reputation to take including in the shadows, or any other run would take the contracts with 'em.
I think that you should create some paranoia atmosphere to the runners and after that made counter propositions of the stoled Mega, that will generate intrigue and possible double-crossin'.

[TheWayfinder]

hope this makes sence :
first off the runners did not steal the prototype or the files .
someone els did and used the runners as his tools to do it .
as long as the runners are in possesion of the "item" thy will be hunted down but once thy have dropt it off thy are in the clear .
that is imo the mindset of the corp . ( heck your next johnson might be from that corp )

of cource like  TheWayfinder said you olso got the people that work there to consider :
did you kill a guard ?
oops your bad it turnd out the be a lover of a big shot and now he is out for blood.
the guard was on the take/member of a gang and now thy are looking for you becous you cost them a resource.
the head researcher/securitie was demoted becous of the theft and now has a bone to pick whith you .
you might not have been the only team after that device and now you are responssible for another team losing reputation .

Just because the runners drop off the item doesn't necessarily put them in the clear.  All they've done is drop off the item, but that doesn't mean the people who are after it will stop looking for them; because they are a link in the chain.  You could have the corp's forces, whoever they hired or whatever, now change their priorities where it comes to the runners, that they're not to kill them until they extract where and for whom they dropped off the item. 

You might also consider what the person who's trying to retrieve the item's goals are.  Does he necessarily want to retrieve it, or just make it look like the data or the item is secured?  Once, I had a junior exec completely finance a team to retrieve some pertinent paydata using his expense accounts, and in the end, while he did not recover the data, he did look as if he made the effort, and made it look that he succeeded.  Sometimes, it doesn't matter if they actually achieve results, so long as they can make it look like they've achieved results. 

But, of course, there could be ego in the way.  Some people have egos the size of the great outdoors, and they find that a runner getting past their security is a slight to their ego.  For these people, revenge is part of the goal, but I would make such people a bit more powerful in their own right than a jr. exec.  Jr. execs don't have a lot of resources at their disposal, but a senior exec would, and he has the ego to go the distance, to do whatever it takes, even for something trivial, all because he's been somewhat humiliated and this could affect his chance of becoming a Vice President or even a full partner in the company. 

A GM could get downright Machiavellian when making corporate villains, so I suggest that as far as the PCs are concerned, keep it simple.  Part of the game is dealing with the fallout of any run, however the size and scope.  You should not stress yourself making complicated scenarios for your villains to deal with, since the PCs will never see that anyway, no matter how much legwork they do (unless they are part of the corporation and are intimately involved in the daily machinations, which is an interesting campaign in its own right).