Guy kept staring and smiling at me

Yesterday at night I was out, taking the subway and this guy came in the train and from the moment the saw me he kept staring, and smiling at me... What does this mean? I didn't know what to do or say so I just kept staring out of the window... (I know stupid right! I wish I said something), and then he had to get off, and I noticed him standing outside and still looking at me and trying to get me to see him...