Malevolent Grin

I was sitting in one of the "comfy chairs" at Starbucks drinking my Venti latte when a 20-something girl came and sat across from me. She kept staring at me and grinning this malevolent grin. I'd look up, she'd grin; I'd look down again. Quickly, I looked at her and then down and then back up again, whereupon I found her to be gone! The door was behind her with glass walls and she was nowhere to be seen. I don't know where she went or how. I'm tempted to think she was a very vivid hallucination: she wasn't very complicated, but she did look very real. That smile though- I'm not sure what that meant; it was like she was going to hurt me and she knew it. I'm glad she disappeared!