The guy was probably just some guy like my stepDad, who has no memory whatsoever for faces coupled with supreme self-confidence & a general desire to be nice. He is perpetually doing stuff like this, mistaking complete strangers for friends or even family members and embarking on friendly conversations with them. Most of the time they assume he is some guy they actually do know and you can see them trying to place him whilst they struggle to make some sense of whatever he's talking about, until eventually they realise he's just a harmless Magoo-type and disengage themselves from him. No matter how often this happens it never seems to dent his confidence or prevent it happening again.

One of his worst blunders was the time he had to drive 2 of my sister's friends back from a football match. They were about 13 and didn't know how to set the situation right, so at first when he spent the entire time caling them both by the wrong names & talking about people they didn't know they just assumed he was just another typical idiot parent, and it was only after they'd been in the car on the motorway for about an hour longer than the trip home needed that he finally realised by the rising terror in their faces that by now they thought he was some crazy serialkiller abducting them.

It turned out that somehow he'd confused them with the daughters of my Mother's friend and thought he had to drive them home there, rather than the 10 minutes down the road where they actually lived.

tl:dr - sometimes creepy guys are just someone's bewildered but well-meaning relative.