How should I handle this?

This weekend my boyfriend and I were watching videos on his phone when he got a few notifications from an app. He quickly tried tilting the screen away from me and swiped the notifications away, but I saw the name and eventually found out its from a random video chat app. I noticed he kept getting a few more notifications from the app throughout the weekend, but we had a bunch of things planned with friends later on so I didn't want to make a big deal out of it and I pretended like I didn't see much. A few weeks ago he told me he was deleting his snapchat because he never used it and it was just taking up space. Yesterday he even made a comment about how he got an email from snapchat about how his account was officially deactivated, but on Saturday I had also seen him open snapchat to scroll through and quickly close it. He's never given me a reason not to trust him before and he's always been a great boyfriend, so I'm rather shocked and not sure how to approach him about all this. I noticed that I started to make some sassy comments to him about keeping secrets and just our relationship in general but even then I couldn't keep it up and played it off as a joke. I was thinking of waiting for a while and see if he's still trying to hide things like this to then say something but that might not be the healthiest option. Even then, what do I say?