If anything is missing I will always call the band and ask them to check (if they already left) and so far no probs, I always get my stuff back. Either way the studio has cameras recording everything, so at the end I'll find out (never had to go that far...yet). Most of the time will be drums parts, cables, DIs, etc.
Off course those thing can be taken by accident....a mic is a little bit harder. But if you ask polite and without an accusing tone. They wont take it bad. (unless they stole it)
Good luck and hope you found it soon.