Alright this won't sound great but he might be just horny and sightseeing you know?! It seems you are together a while, kinda stable, that can be scary maybe he is checking himself if he really is in for life or he still wants something else. People have this doubts all the time, we are not robots. Just be honest, talk to him, be honest say what you found, where and how you found it, if you were snooping you definitely have doubts yourself, so a good conversation will do you both good. I say this because he doesn't look like he is cheating on you, he is probably just curious. I did that as well, never even gave more than a polite answer back but I had a moment of self-doubt and wanted to check if I could still get attractive girls, so I went on tinder and checked if it was as easy as everyone said it was. After understanding that what I was looking for was right there with me I found myself a foul and told my gf, at first she didn't understood why I would do that and I showed her the app and deleted everything in front of her. People have doubts, the important thing is to be honest, doesn't mean love is gone, it might just be dorment and need a little shake.