This sort of thing is an arms-race, and if you don't want to spend your life on counter-surveillance tech and opsec, you're not going to have any certainty. Pu

This sort of thing is an arms-race, and if you don't want to spend your life on counter-surveillance tech and opsec, you're not going to have any certainty. Publicize, loudly, cases when someone violates your trust. Because that's what this reduces to; you're trusting whomever you're paying for that room. If they don't deserve that trust, ruin their income stream and they'll stop. Applications of this approach to less-vulnerable violators left as an exercise, etc.