I dont want to put any thoughts in your mind or indicating that husband is doing something bad but just want to share my experience. My husband who has a great public impression used to do that then obe day I found about his porn addiction and after a year when he started doing it again i found about his prostitutes addiction.
A couple is supposed to have this sharing-everything-that-comes-across routine even if the thing is a taboo in the society or is considered vulgar. If a couple maintains such an attitude, they would be going miles together but if somehow there is a communication gap about anything any of them has ever done or watched or read, there is bound to be confusion and oblivion. So, I would suggest you to go and talk to your husband about everything you might not have conversed about earlier. Express yourself, talk about the most controversial topics. Or rather become teenagers again. Now, I would not suggest you to bring in the worst possible scenarios. You need to have more conrete documents/proofs. It is a human trait to hide the windows screen when someone else enters the door. You should focus on making him feel that you are her partner in life, you should make him feel the confidence in you so that he may tell you about his secrets. You can start by sharing your own mischiefs with him.