How Much Privacy Is Secrecy?

My fiance and I have a pretty open relationship, or so I like to think.

However, he is weird about his phone. If I playfully take it away from him, he gets very agitated very quickly.  He keeps it password protected.  I've never asked to go through it, or tried to, but this behavior makes me suspicious.

I'm aware he's entitled to privacy, and that parts of his life don't concern me.  And honestly, if he didn't get so weird about me touching or playing with his phone, I wouldn't be concerned at all.

My phone sits around all the time, unlocked.  There's no password to get into it.  It's just there.

If he wanted to go through it, he'd find a hundred pictures of our dog, a bunch of selfies, and some texts to my mother.

I often wonder why he's so defensive about his phone.  He doesn't strike me as a cheater, but then again, do cheaters ever?

How much privacy is too much?  When does it become secrecy?