I wouldn't leave it out. To much access to your personal information for anyone to pick up. Including residents. Unfortunately the answer maybe to keep thing's you do not want others to see put up. You can not control other's but you can control what you allow.
I do not thinking calling HR is the first choice to resolve the problem. Speak with the employee and your direct supervisor and let them escalate. Rule of thumb.

The behavior is totally unacceptable and should be reported to your manager and HR. Keep your phone in your purse. It shouldn’t be out in the office.

As a Regional one of my Managers did the same thing. I added and have kept a lock on my phone. It's just rude

I've been with my man for 2 years and I havent even looked at his phone!!!! Much less a coworker? I sweearrr classy is hard to find lol

That’s a violation of my personal property and space. That’s like somebody looking in your purse. There are some deeper seeded issues! Yikes!

I would be angry, breach of trust. Enable password to unlock.. Trust no one. Simple rule, if it isn't your's don't touch!