The first thing you should do is to talk to her and say what you have just said here.

Yes, she is being irresponsible by letting a stranger stay there while she is out.

She is putting YOU in danger and you have a right to say something about it because you fear for your saftey.

You also have the right to feel secure about your property and pocessions while you are out, and her leaving someone (anyone!) who does not live there, alone and unsupervised, is not smart.

Hopefully if you talk about this with her she will use better judgment. Unfortunately she may not see it the same way as you do and will not change her behavior.

What the heck does he need to be there for if she is not even there?!?!?!????

Guests ahould always be supervised by the person who invited them and not left to wander about and left to their own devices.

If you signed something to live there you may be stuck there for a while. Get a lock for your room and lock it before you leave and maybe you will feel safer locking yourself in your room while a stranger is there. Get some pepper spray.

I'm w/ Bud--talk to her.

In fact, this should have been a big topic of discussion and agreement before you became roommates.

You should have decided, together, what would constitute an acceptable guest (old friend? sister? new boyfriend?), and what would be appropriate: how long can they visit? overnight? advance notice? can they have keys?

I know when I was a roommate, I didn't feel my roommate's old friend needed to be "supervised" by her. But we also had a hard discussion about how long she could visit--we both feared, I think, that she'd come to "visit" and stay for a month.

We also decided that NO ONE could visit, esp. overnight or for more than "stopping in for a bit in the day" without advance permission from the other roommate.

But her letting a male stay there without permission from you is out of line.
Her letting someone she knows so little about, stay there when she's not there is out of line.

(actually, your safety would be compromised only if you WERE there--him being there when you're out doesn't endanger YOUR safety; just the safety of your stuff)

I'd be really alarmed that she was letting some guy she had dated "a few times" be in YOUR home, without you having said, "OK w/ me."