I’ve always understood that it was St. John. Mark’s Gospel says “a young man”. The young man present would have been St. John. He went home, got clothes and rejoined Christ.
Another thing that is not mentioned in Mark’s Gospel is that Jesus turned around and looked at Peter after his denials. So Christ and Peter must have been in hearing range of each other.
“I heard it was St. Luke. St. John was a man and at the Crucifixion.”
Actually, I’ve heard both of these put forward. Of course, both are apocryphal.
I think that I remember discussion in “The Lamb’s Supper” by Scott Hahn, of it being St. John, “the apostle whom Jesus loved”. St. John was the youngest of the apostles. I sometimes have seen him projected as an adolescent, or as Mark’s reading states “a young man”.
Anyway, it’s interesting stuff to ponder.