When I'm at work the red flag for me is: What time do you get off work?
I just met you. You're a customer who hasn't even exchanged so much as a greeting.

I work in a library where we have to sit out in a public area for at least part of the day. Yesterday, I watched a patron I'd never seen before walk up to my co-worker's desk, and ask her how she was doing. She said fine, then asked if he needed help. He said no, that he was just asking how she's doing. She answered fine a second time, and if he needed anything, so he must have put her on red alert somehow. He's standing too close to the desk where she's sitting trying to loom over her. I'm sitting behind a higher counter about two or three feet away, and he hasn't noticed me, because he only has eyes for her. After her second rebuff, he starts to walk past my desk, sees me where he didn't expect to see anyone, looks me right in the eyes, and says nothing at all. No smile, no greeting, nothing. Quickly walks in the opposite direction from where I'm sitting.

That was a huge red flag to me, and makes me wonder what he'd have done if he hadn't noticed me. This is why we usually have the policy of at least two people on the public floor at all times.