The question there is then how you know what you're seeing. I doubt a smartgun is going to be totally obvious as such at a glance in a crowd (less of an issue if your team and your enemies are the only icons within 6 blocks in the barrens).
I like to think of it from the other way....
If you see someone wielding a smartgun then you automatically also see its device icon.
Unless if he is wielding a smartgun over 100 meters away... in that case the device icon is as obvious to spot as a neon sign but you still need to concentrate for a second or so to spot it in the sea of all other devices.
If his smartgun is running silent then you still need to concentrate to spot it in the sea of all other devices, how obvious it is to spot depend on how good the owner roll in the opposing test, but on average you need about two hits (one net hit) on your opposed test to spot it (so it would be as obvious as a street sign, pedestrian, conversation or silenced gunfire).
Also situations like...if you're in a mall and don't have the commcode of your mark you're going to need to look around til something is recognizable and you know you're looking at the right device. That kind of thing shouldn't be instant.
It's really infuriating when this part of the rules is so vague but other parts are needlessly/valuelessly granular
Well - What
do you know about your mark?

You have to know
something or else you will never know that you got the right persona.....(!)
If you know how he
physically look then you can take a Normal Perception + Intuition test with a positive dice pool modifier of 3 dice because you are specifically looking for your mark that you know what he looks like but you also probably get a negative dice pool modifier of 2 dice because there are so much people (you can also substitute your Perception for Sneaking).
If you successfully spot your target then you automatically also spot his persona.
Unless you spot your mark over 100 meters away... in that case the persona icon is as obvious to spot as a neon sign but you still need to concentrate for a second or so to spot it in the sea of all other persona icons.
If his commlink that he based his persona on is running silent then you still need to concentrate to spot it in the sea of all other persona icons, how obvious it is to spot depend on how good the owner roll in the opposing test, but on average you need about two hits (one net hit) on your opposed test to spot it (so it would be as obvious as a street sign, pedestrian, conversation or silenced gunfire).