If you know at least one feature of an icon running silent, you can spot the icon
... the first thing you need to do is have some idea that a hidden icon is out there. You can do this with a hit from a Matrix Perception Test;
asking if there are icons running silent in the vicinity (either in the same host or within 100 meters) can be a piece of information you learn with a hit.
If you already have information on the icon running silent (assuming feature & information are interchangeable), then you can attempt to spot it. Further assumption would lead one to believe that if this information likewise does not produce a unique result, the icon you attempt to perceive will be chosen from the list at random.
There was a brief time when hackers thought they could confuse security by flooding hosts with dozens of RFID chips running silent, but once they figured out that demi- GODs knew enough to design their scans to screen for icons that were running silent and were not RFID chips, the days of that trick were numbered.
That’s the part of Matrix security that too many people overlook—it’s not about just looking at reality, it’s knowing how to define reality so that what you want to see comes to the fore.
No matter how you twist it, you can't use a bag of RFID tags against corporate security. Whether you can use it against other matrix entities is another question.
by the logic you're suggesting, trying to drive a motorcycle across town would be suicide.
When a character is piloting a vehicle in non-combat, or everyday situations, no test is required (unless the character is Incompetent, and then hilarity ensues).

Okay, the flavor text is nice, addressing the specific issue, but that doesn't illuminate how you actually counter that approach. The actual rule seems to state that you can't know what the icon is without inspecting it, and you can only choose which one to inspect at random.
Until then, they just know there are silent icons in the vicinity.
Per the Matrix Perception rules and the Running Silent rules, this seems to hold true. You have to scan for icons that are running silent -- once you have done that, then you roll an Opposed Test vs that icon to interrogate it, one net hit for spotting and every one after that for analysis. But you have to do that
for each silent icon, at random, one at a time.
Really not debating the flavor text -- I just want to grok how this would work. It's all well and good to say "filter out the RFIDs!" but until you've done the above, you wouldn't know what is an RFID and what isn't.
A non-Decker isn't going to stop a dedicated hacker. But I don't have to outrun the bear, I just have to outrun the guy next to me. My logic is, if the Decker has to individually mark each icon on my PAN, and my Agent is scanning along like Patrol IC and then rebooting the PAN any time it sees anything that isn't supposed to be there, they're either going to move on to an easier mark or they're going to get tunnel vision while our team's Decker hunts him down and stakes him out for the ants.
As for the motorcycle thing, that just illustrates the point -- in an everyday, non-combat situation, you're not going to have to worry about Critical Glitches if you ask your Agent to keep an eye on your PAN for you.
-Jn-