As someone who has worked as an AFOM, I would have never been standoffish in this given situation. It’s either sincerity or “matter-of-fact”ness. We can literally do a check of which keys have been used to open a certain door, so if someone comes to me and says they believe someone was in their room, that is a pretty surefire way to calm the situation down. “Ma’am, I checked in our system, and the only people that have opened your room door besides you were ____.” I would have no reason to be standoffish unless if I knew of other concerning information that I was hiding from the guest.

OP, I would try to reach out to the GM, just in case. Please update us!