I'd calmly tell him you saw what you saw and ask him if he feels the need to talk about it or come clean with some things. He may not say anything, but I think it needs addressing. If you can muster all the kindness and calmness maybe he'd feel better about talking to you about it.
I caught my XH googling his now OW 4 years ago. I never said anything about it and I wished I had. It's possible he might've come clean about their relationship, but possibly not. Given he went balistic when I caught him at her apartment when we were still married, and calling him on her being seen at his apartment daily while he was having me and the kids for an overnighter.
Nip it at the bud, I say. Just be calm and hopefully you can draw him out to talk about it.
