ask him to reopen it then and there
If you feel like it’s sketchy it’s probably sketchy. The only good thing it could be would be him planning a surprise for you but I’m gonna assume he’s not that type of guy if you’re tripping about his phone.
I find when they are not very specific it means they are probably lying , he should be able to have his phone out and not like this hidden treasure with his secrets, id ask him to see his phone and to be more open, if not...sneak it. Try first though, and if he cant give a reason why..yeah I'd be suss
Always your gut.