What does it mean when a guy goes through ur phone?

its a fwb type of situation, and he took my phone and started looking through it. its not the first time hes done it, but of course i didnt want him looking through my phone and asked him what he was looking for and he said he didnt know. why would he want to do this if he doesnt want to get serious. i never wanted to look through his phone because i have no interest in what i might see