It would look like this:

rooms: 
- name: Your Apartment
 paths:
 E:
 - room: Outside
 actionGuard: foundWallet

If the "foundWallet" action guard fails, you won't move to the room to the east.

One thing that we might want to consider is that for action guards like this, there might be some need to "destroy" them, which I'm afraid is complicated to implement but basically means that if they pass once, they should never test again.