Guys, just some food for thought:
Wouldn't the things being ruled as they are turn things a little bit too easy for hacker/'mancers? For a primary hacker/technomancer, it will take only two or three rolls to enter any node, and it's really, really hard for one to be spotted on his first turn. So, if one triggers an alarm and has the mute option, virtually all the time he will just turn it off afterwards and all is green and cream. Mute, all things considered, seems really overpowered.
I have no problem with this situation being ruled as it is, but I would suggest that, for the next occurrences, at least a Matrix Perception Test should be successful for the hacker/'mancer to know that he triggered an alarm (alert status of a node is something that an MPT reveals).
Anyway, just my opinion.