Why did you snoop in the first place? Was there something he did to make you suspicious? Were you looking for a reason to start an argument?
Think about this - if you confront him about this (if it is so egregious to your moral code that you feel you MUST tell him about seeing it), then be prepared for the argument to quickly shift to why aren't you trusting him. Which is the more important argument, actually. You say you have nothing to hide from him. Maybe he feels he has nothing to hide from you either. But there are some things that every person does on his or her own. Do you invite him into the bathroom when you are dropping the deuce?
Good luck.
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