Rumours

There was a rumor of a mysterious man that roamed the hall of hogwarts.
He was too old to be a student and yet too young to be a teacher. He wore muggle clothes and whistled as he walked with a little pep in his step. People who interacted with him said he was awfully sarcastic and almost always had a mischievous smile on his face.
His shaggy black hair, light olive skin and dark eyes reminded people of two students in 6th year, but not enough to make an impression.
But all they knew was that he was a strange lad, and he was NOT from 1976.

OR

Harry accidentally goes back in time and forces james and severus to get close.