He is working in the same office building but by a different company.
He welcomed me sometimes already, and it was clear he remembers me from somewhere. Also, I remember his face, however, I can't remember, where did we meet.
I welcomed him back, trying to hide that I don't have any idea, who is he.
The problem is that if he finds that I totally forgot him, it wouldn't make him the best impression. Thus, I would like to avoid it.
However, I am very, very curious, who could he be. I think I should somehow find it out. However, I have no idea, how could I ask it from him, because asking this, I would reveal that I've forgotten him.
P.s. It is Germany.