It's like finding a bra and panties (that aren't yours) in your boyfriend's room. Sure, MAYBE there's an innocent explanation (like they're cross-dressing), but you'd be an idiot to not at least suspect that they're cheating. And the more they gaslight, pretend there were no bra and panties in the first place, and and act like it's you who are crazy for even thinking they're cheating, the more certain you become that they are.