Personally I don't think your warning flags going off are not unjustified in my opinion. It sounds as if this is new behavior from your bf. I'm assuming by the way it surprised you that he hasn't done this before and if that's the case, it is something to be suspicious about. What, if he has locked his cell phone for fear of other people getting access to it other than you, frankly, he would have been up front about it. At least if it were me and the reason was justified, I would have done everything to ease your mind with the explanation at least but he snapped at you which is a bad sign. Secondly he didn't offer to let you unlock it for him, which, it also sounds as though you would normally expect him to give into. So if it's not some other reason for locking his phone, it does sound like it could be hiding something from you IMO. Of course this is just my opinion on this so.. At least though it is something that you need to look into further. I would go with your intuition on this and talk to him. Depending on how he reacts to your asking why he locked it and why he won't let you just enter the password, I would judge from that. For me personally, the one I am closest to, my SO, should have nothing hidden from her. But that's just me. Again, if this is not new behavior but something you didn't know about him it might be ok, but if it's new behavior that's changed, you just might have something to worry about. I'm sorry but that's just my take.