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It's possible that he could be hiding something, but in all likelihood he just wants to have some privacy and that's fair enough - it is his phone, after all. Most likely he just wants to have some healthy boundaries, and it's nothing to worry about. Most people have things in their life that they'd rather keep to themselves just for their own comfort, rather than because they're necessarily hiding anything or that they don't trust you - you probably wouldn't want your parents rooting around in your bedroom even if you have nothing to hide from them. It's the same concept as that. As long as there aren't any obvious red flags (being generally secretive, trying to avoid you etc.) chances are he's probably just wanting to keep his private stuff to himself.