As we all know, the 'rules' are whatever a TSO says they are.

That said, a supervisor at PHX T2 told me that TSA has the right to inspect any bag, any time, and is under no obligation to inform the pax or allow the pax to witness the bag search. This was after I expressed surprise that my bag had been searched and my BP and ID removed and copied behind my back (literally) and without my knowledge while I was getting groped.