naivete

At work today, there was a pre-Thanksgiving/extended celebration of the Day of LisaM. potluck.
When I went to break there was this guy in the break room, a guy who I didn't recognize. Granted, I barely recognize half of the people who work in the other rooms of our lab, but I was really weirded out by this guy. Not only had I NEVER seen him before, but he set my alarms off in a big way.
Our breakroom is in a secure area and I had to put the code in to get in, so naturally I assumed that he got in via code as well. He was talkative and looked like he knew the place inside out, he knew where the get towels, he knew where we kept the spoons, he just acted like he belonged there.
So we're making some small talk and the whole time I'm thinking that something is amiss, so I ask him where he worked and he just brushed that aside and started talking about his shoes.
But I just kept thinking that something was off and so I paid really close attention to what he looked like and how he acted, I even thought that he was a tweaker and probably on meth because he just gave me that vibe.
After about ten minutes he left. So I just sit there dumbfounded, wondering what the hell just happened and who this guy was.
After my break I walked over to the supervisor's offices and saw him walking out. I wondered where the plate of food he had went, he couldn't have eaten it that fast. But he came out of the supervisor's office, so then I figured he must work with us because why else would he have gone in there?
As soon as I got back into the lab, I asked around...hey who's the guy with the goatee and somebody said, "Oh that's Dan."
Mystery solved right? Not even thirty minutes later, our manager has called security because somebody walked into her office (she left the door open) and stole a bunch of stuff.
And apparently I wasn't the only person who saw this guy and thought "What the fuck?" but I AM the only one who made conversation with him and saw him in both areas.
So I'm either naive, though the fact that I was suspicious kind of puts a small dent in that theory, or I am a totally jaded city person, the kind of person who would hear and see a person getting raped and murdered and not do anything about it. I'm a little scared either way.