They can "dodge" them. But they have to know they are there.
I wouldn't quite say that. That sets a precedence that the player must know about each and every possible thing that could percieve him/her in order to infiltrate successfully. That makes stealth unreasonably difficult, as well as somewhat unrealistic.
I disagree, politely, with you here. It's what the whole legwork phase of a run is for. There can be the; "Did you get the security schematics from the contractors about that facility?" "Yeah, chummer, the room before our target is a laser sensor matrix, vertically and horizontally, with only a two millimeter gap between the lattice work." "Frag, we better get our hacker to take that out." Or the; "Hey, GM, this is the last room before our target, do I see anything?" (rolling perception) "Nope, looks all clear." "Okay, well to be safe, I'll use Infiltration." GM to him/herself, "well, he can't squeeze himself into a <2 mm spaghetti strand so the alarms go off."
Inflitration is the skill used when a character wants to sneak around undetected by either other characters or security sensors. (SR4A, 124)
Then
Technical security includes alarms, sensors, scanners, locks, and automated systems. (SR4A, 261, original emphasis)
Trip beams are...perimeter alarms. Noticing [them] requires a Perception + Intuition (2) Test for visible beams, or a threshold of 3 for infrared beams. ... Squeezing past a trip beam maze requires an Agility + Reaction Test against a gamemaster-determined threshold. (SR4A, 261)
If you look through the whole Technical Security section (SR4A, 261-264) it modifies, clarifies, and specifies how Infiltration is used under certain conditions. Like not at all for trip beams or, as for motion sensors;
Defeating a motion sensor requires that characters move very slowly through the field, one half-meter per Combat Turn, and succeeding in an Infiltration + Agility (3) Test. (SR4A, 261, my emphasis)
So if a player doesn't know that the motion sensors are there and says, "hey, I'm going to use Infiltration to move across this room" and then proceeds to move 5 meters in the first Combat Turn...no matter what they roll on their Infiltration test, even 50 hits...they're getting picked up by the sensors.
Knowing is half the run.
You don't need to know where a camera is to know that there probably is one and you should keep to the shadows. You don't need to know where each guard is to know to keep out of open areas and stay low to the ground, etc. And you don't need to spot a laser trip beam to assume that there might be one. Of course, knowing helps improve your task, but not knowing doesn't make the player worse at sneaking. That would be applying a modifier incorrectly (a penalty to player when it should be bonus to observer).
But...the cameras could have night vision or thermographic sensors linked to motion detection software...not knowing gets you geeked. But there could be a guard leaning against the wall just around the corner and you'll both have to roll surprise and perceptions...not knowing where the guards are gets you geeked. And sure you can assume there's a beam, but where do you duck, step over, slide under, jump over it, is it a meter into the room and a meter off the ground, two and one, one and two, 2.72 and 3.14?...not knowing gets you geeked.
If the runners didn't check the security out and find out where these things are then they're walking right into it. Of course the GM should, through varying levels of difficulty, make this information available to the characters with proper legwork on their part.
This becomes an extremely important point when you consider astral observers. If you have the mentality that you must be aware of something to sneak by it, then any mundane automatically fails against an astral observer. That's somewhat absurd if you ask me, as it renders an entire character concept ineffectual and useless. The Astral observer should of course receive a good bonus for aura on shadow contrast, and possibly negative background count (low life area such as a city would have a -1 Background Count that becomes a +1 Astral Visibility modifier, check SM for the chart), likely around +3 to +5 dice on average but is very situational.
This is why A ) mages are rare and B ) kill mages first...before they get to work. A mundane trying to sneak down a sterilized hallway where an astrally projecting mage is hanging out at the other end...game over. It would be like not noticing a dancing Christmas tree coming down the hall way. But a facility that can have mages hanging out 24/7 at the end of hallways is, well, not realistic (unless we're talking big big budget for funding, like Lofwyr's deltaclinic for his drop bear army). It's then left up to watchers and other spirits which are either very unreliable or expensive to maintain.
But something to consider is a perception bonus for superior positioning. A vantage point over a bottleneck, a camera in a good place, or something like placing a guard at the end of a narrow hallway that gives the guard a superior view of that hallway. This bonus could be offset by the player making their own perception check to notice the difficulty and plan around it, giving them a similar bonus or taking away the NPC's bonus depending on what they do.
Agreed

Also, the rules state that some things are just plain obvious and do not require a perception check to be made. An example could be a camera that watches a door and does not pan. If the infiltrator tries to use that door they will be seen, it's just impossible to do it without being seen.
Agreed. Sneak up on a guy, sure. Palm his pistol with out him knowing, okay. Firing the gun at his friend (or just firing it for that matter), silenced/suppressed or not...nope. If the guards within 10 meters, he/she'll know exactly where it came from and look. Lots of videos on youtube of people firing silenced/suppressed weapons (some even with attractive members of the gender you might be excited about, you choose the adventure

) they're definitely far from silent. (Of course, with cinematic rules, hell, why not

).
I think it's this last situation that ARC was speaking about, I just don't want anyone coming away from this discussion thinking that a infiltrator must know where each and every opposed observer (NPC, sensor, or otherwise) is in order to infiltrate successfully.
And I, politely (at least my intent has to come across as polite), disagree for the reasons above. And I would go so far as to say that a player must designate who or what they are trying to sneak past before they roll Infiltration. You have to perceive where the eyes are, and aren't, in order to know how to effectively avoid them, meat-body or otherwise. As a completely absurd and, hopefully, funny example...ever watch a movie with a guy sneaking around. Despite all his best efforts, even if the actor studied with ninjas for a decade on how to be sneaky, why does the audience always see him?
