As someone who has cheated in the past I hid my phone. After I stopped being that person I still had the urge to hide my phone. This wasn’t because I was cheating but because I was used to it.. I was still hiding certain interests on insta etc but that was just out of embarrassment. I believe some people are hiding their phone out of privacy rather than maliciousness, he might watch porn etc and want to hide that.

In terms of looking at past pictures with exes this only causes paranoia and makes you feel sick, I would avoid this unless you can feel at peace with the idea of his past relationships.

You will never know if he is cheating or not unless you find evidence but I wouldn’t base past bad experiences as evidence for your current relationship. See him as a individual. I would make a list of the reasons you think he’s cheating, I would then make a list of reasons he wouldn’t cheat eg. how does he communicate, how much time does he want to spend with you. This might help create clarity and peace of mind.

Your not alone in feeling uncertainty in this difficult time and I hope you find happiness