Um... don't they have to get a warrant for that sort of stuff and doesn't it have to be served to you?
Nope... Not anymore. Anything left with the bank is considered their property until you retrieve it. The bank can give access to the box at any time to any one claiming to be a "law enforcement" agent or a federal agent. Normally they don't even need to provide identification that they are an agent. The bank nor the agent do not need your express permission to open it.
Rule of Story: DON'T USE DEPOSIT BOXES.
Same is true for storage lockers.