Two silent running Personas arrive at a place in the Matrix (say opposing Deckers, trying to get to the same device). This place is NOT a host.

How would they detect each other?

1) They are both outside of each other's 100m physical wireless antenna range. (p235 left column)
2) They don't know "a feature" of each other (p235 sidebar)
3) They can't see each other's Personas as they're running silent.

What makes me think that you cannot find each other feasibly is this piece from page 235 on Running Silent:

It refers to that 100 meters around your physical location again, meaning Matrix perception is not a 100m AOE scan of wherever your Persona is at the time.

Hence, how could you detect each other out in the Matrix?

You can't, I guess, unless you have a reason to believe there is a hidden icon, like an alarm going off suddenly for no reason, or a file you were accessing is edited or you notice with a Matrix Perception test that a file was just copied.

I believe this is designed to protect you as a hacker, rather than punish you for not knowing you're about to get into a decker duel.

So by inference, by seeing Marks appear, items get bricked, you know there is someone there. I would argue you could then just start rolling Matrix Perception against their Logic+Sleaze, right?

Yes.

p241
If you’re looking for an icon that is running silent (after you’ve determined that it’s present), the test becomes an Opposed Test, with the target de- fending with Logic + Sleaze.

You can be pretty sure its present if something gets bricked or a file is just edited. Or that's how I'd interpret it.

\o/ Understanding was had!