How do I know you?

Two in two days? Woooo Hooo!!! Have you ever met someone that you know you have met before but have no clue where? This happens to me quite frequently and drives me mad until I remember how I know this person. I am currently still stewing over two people one I have know for almost 4 years and I know I met him before but neither of us know where from. Sometimes I think he is toying with me and really knows but its something embarrassing so he doesn't want to tell me, other times I think he just isn't as obsessive compulsive as I am! The second I met yesterday and I said you look very familiar to me. I apparently did not seem at all familiar to her. She began to ramble off ways that I may know her... Nope. This got me questioning how would someone know me? I am not generally outgoing. I don't join things! Are you defined by what you do? Its almost like a list of major accomplishments in ones life. If someone thought they knew me I would mention HS, my family members, my employers, and maybe friend groups. But this is also a lot of who I consider myself to be! I am a large reflection of who I surround myself with. So who are you think about how you would guess someone might have met you and then think if that might reflect who you are? Am I the only one?