Glad you are getting some help from the BM … although the BM may or may not have told you the truth. Sometimes a BM has a boarder that they sort of ‘protect’ for whatever reason. Or they ‘protect’ all of the boarders because they don’t want the boarders to have bad feelings for each other.
One approach is to take a tone that maybe a possible thief picked up that thing “by mistake”. Explain that you “lost” yours (as if you are taking some blame for losing it) and say “Is there any chance those could be the ones I am looking for, do you think?” Since you don’t wish to be confrontational, be crafty instead. 
I look on a board barn as like a sleep-away camp, when it comes to things. Label, mark, label, everything. I keep a sharpie in my grooming box. And lock or at least shut away out of sight anything you don’t want used by others. Even if it’s a great barn with no issues, the next new boarder could be the one.
An out-and-out thief is almost the easiest thing to deal with. Catch 'em, get proof and get them kicked out. True thieves are usually not as good at concealing what they do as we assume they are. They don’t change their stealing ways and will even take things they don’t need, even though if you knew their history you’d be surprised at how often they’ve been caught at it.
It’s the “borrowers” that are the real ongoing problem, IMO. The gal who just graduated from high school, first time in a ‘real’ boarder barn, who thinks that everyone can use everything, just like at her parent’s barn at home. Or the lady who is always going to buy you more fly spray but never will. Or the chronically disorganized who can’t find their own stuff, from hoof pick to leg wraps to shampoo, and has to leave in just a few minutes so doesn’t have time to look for hers but will just this time use yours since yours is organized and easily found.
Etc. Those are the tough ones because they aren’t clearly thieves, but they don’t change, either.
For your really valuable stuff, a locator may be just the thing. There are some caveats on how they are used (they are not GPS and locate only within bluetooth range). But they will map to an address where the thing was signaled last (you’ll know if it is on premises or not). And the “Pro” is the only one I can really hear from across the room or if the Tile is within fabric or stuff.
https://www.thetileapp.com/en-us/products?gclid=EAIaIQobChMIoMuG79np4wIVAhgMCh2Pyw8PEAAYASAAEgJbkPD_BwE