I'm sure it happens. According to what I've seen on TV, all you need to do is stick a thumb drive in a laptop and utter the magic incantation "OK, I'm in" and you can either download the entire content of the network or implant a virus in the time it takes for someone to come back into the room.

Unlikely, given that it does not happen in commercial companies either and there no one particularly has reason to care.

Private companies are compromised by visitors every single day. Outsiders on-premise are one of the largest attack vectors after insiders.

This is why both the local network and endpoints are no longer trusted in security models.