There's not really a way to "break in" to a commercial aircraft parked at the gate. In fact after some inadvertent slide deployments at US, it became procedure for a designated FA to make sure all the slides were disarmed before leaving so the cleaners / caterers / mechanics could open any door without getting hurt (the only ones we kept armed were 2R on the B757 and B727).

The dirty little secret in the airline industry is the widespread habit of people helping themselves to booze to take home or on an overnight (Google "crew juice" ). It sounds like in this case someone really overdid it, and the Station Manager called in a sheriff's deputy so the word gets out in the system for everyone to dial it back a bit.