In what state?
Your argument is likely irrelevant since any claim you might have would arise out of either negligence or willful/criminal conduct of an employee. That's exactly the point of the "signed agreement that . . . absolves [the building management] of liability if anything goes missing." Liability wouldn't exist without at least negligent, so saying that the concierge's conduct was "pretty clearly negligent" is beside the point.
Depends on the exact wording of the signed agreement and the laws of your unidentified state.

