Deep thoughts.... How many Crown Royal bags have you 'freed up' this week? :smile:
As a matter of good practice I wouldn't put an expensive mic like a U87 in harm's way by hanging around moderate-high SPL if it weren't in use. Even if it was in use, it had better be earning hazard-pay (enough to replace the element) to justify the risk.
But in your situation I would think it's mighty quiet inside that safe. I'm betting the wear and tear on a stored microphone is beyond negligible - especially latched inside a form fitting, foam lined, cherrywood box.
So unless you're down-range from an artillery site, (or TheJackAttack's place) you can cross this off your list of things to worry about. I'd continue to worry about dust, , and moisture (and rough handling).