I'd have to mention it, if it were me, because I can't bear not knowing and the suspense. I'd rather hear bad news than no know.
I think I would laughingly say something like
'You know how you're not very good in hiding the websites you've been on, guess what I've found that you might not have wanted me to know about?'
And then he will either mention it and explain, or won't and you can tell him what you found.
You will have to gauge the conversation and how to take it forward.
Hopefully he will instantly be able to explain it all and it will be fully convincing and make sense for you and reassure you.
If there is any doubt, awkwardness or suspision, I might say
'I felt a bit like that character in 'Love Actually' who finds the necklace but gets the Joni Mitchell CD. Is there something you need to tell me?' - the tone here could be very light, or pretty serious.
This is just how Inwould play it.....but I'd have to push to know. Of course if he is having an affair and is determined still to hide it, he will try, but if he is, this will be a good opportunity for him to admit it.
Depends if you really want to know or not. I'd have to know because otherwise doubt would eat away at me....not a good place to be in.
Really hoping it turns out to have an innocent explanation OP. Please let us know.....we will all be rooting for you.