Noticing someone

I have found in life that when I am out in the public (at the store, etc.), there are certain times I notice people who stick in my mind. I don't know them from adam but yet, they linger in my mind. I don't know if it is how they act, the way they carry their self or what it is but I notice. It is long after the initial contact that I think of them. Do they need a friend? Did they have a recent traumatic experience? Are they happy? I don't get it but I do notice them.