If that's your only evidence, I'd look at the relationship as a whole apart from this single incident. Is he generally trustworthy? Do you know any of his former girlfriends, and if so, how do they regard him? Is he open about letting you check his cell phone, or computer? Not letting you in the room is not a good sign if he had nothing to hide. If you see patterns like this, you might, unfortunately, be on to something.