The strange person

When i was a child during my visiting to my grandparent with my mother she told me if you want see something is strange just when your grandfather is sleeping just look at his chin during the darkness.
Indeed i wait that and when i entered to his room to see him during his sleeping i found the room was dark except small light spot comes from his chin and i closed to him to know why his chin look like that.
I ran to my mother and told her about that and i asked her why his chin look like that.
She told me when my grandfather in his job and he came back to the home after midnight and during his walking someone met him to shake hand him and asked him if he is good or not.
My grandfather asked him who was this person and he touched his chin during that but he didn't remember this person and he continued his way to his home.
But my grandmother discovered this light spot on his chin and asked him about that so he told her this story about this strange person.

So my feeling about my grandfather like a box of secrets and he kept with many secrets even his death to die with him.