What a Small World

Today on my way to work, I parked my car and walked toward the elevator. I saw someone I know from work and tried to wave at her. She didn't see me. However, there was another woman who did happen to notice me wave. She thought I knew her and held the elevator for me (I didn't want that to happen since I hate when people hold the elevator).

I walk into the elevator, and wouldn't you know it, I did know her. We went to college together at Princeton. She was a year below me, and we had a mutual friend who we both keep in contact with. I haven't seen her in about 5 years though. Turns out she works in the building across the street. What are the odds that I would wave at someone, she thinks I know her, and I end up knowing her?