Someone locked eyes with me from across the room (same gender) at a simcha and waved vigorously looking for a response. I had absolutely no idea who this woman was, but from her reaction, felt as though I should. I nodded to acknowledge her, but clearly she did not get the response she was looking for. I felt so terrible.
I looked for her later to say hi and try to see if I knew her, but she had already left. Oh, well.