It’s a gaslighting attempt.

He keeps discovering moisture in his bag, he’s gonna go looking for causes. No causes seem to make any sense, so then he starts thinking maybe bags are always moist and he never noticed before. Maybe not exactly this path, but it will be some escalating set of irrationalities developing as he tries to reconcile what he’s seeing.

Even when a person knows they’re being gaslit, unless they manage to escape the situation with concrete action, it will still degrade their sanity.

In this case, he was lucky enough that she was doing something that could be caught on film easily, and he was fortunate that he actually took the steps to figure out how it was happening.

In other words, he trusted his gut that it wasn’t accidental, and then he gathered evidence to resolve the ambiguity. He handled this correctly, and successfully escaped the damage that gaslighting normally causes.