And there followed him a certain young man, having a linen cloth cast about his unclothedness ; and the young men laid hold on him. And he left the linen cloth and fled from them, unclothed. Mark 14:51,52.
[KJV, slightly altered with respect to the matter of clothing]
That Mark says 'a certain' young man, without naming him, is very reminiscent of John's way of mentioning himself without actually doing so. I don't believe that in either of their cases it is false modesty. I believe it is a valid technique when one is obliged to mention oneself in a report.
Mark's account is the only one to include this detail and in this he, unusually, differs from Matthew, from which (it may well be) he drew the majority of his factual data.
It is my own personal surmise that Mark read the book of Matthew and realised that there was another book buried within it, that deserved to be extracted and to stand separate.
But it is a surmise, only.
Mark's account is also the only one to include the matter of the young man sitting on the right side in the sepulchre, clothed in a long white garment. Mark 16:5. And the two incidents are striking in their content and in their relationship to one another.
A garment 'cast about' and then relinquished in flight.
A garment, long (fully clothing) and white (clean and pure) ; and a seated, composed situation on the right hand (available for service).
There is quite clearly spiritual truth to be drawn from these two places regarding the faulty following of a Christ on earth that leads to betrayal and shame - and the faithful following of a risen Christ that results in composed, restful faith and an availability for fruitful service.
We see this also revealed in Mark's own life as reported in Acts and in the epistles. Returning to Jerusalem, he 'went not with them to the work'. And caused a permanent rift between Barnabas and Paul.
John Mark (or 'Marcus' for his name is a Roman one) was Barnabas' nephew, the son of Mary, Barnabas' sister, Acts 12:12,13. And associated with that house was a damsel named Rhoda who answered the door when Peter was released from prison by the angel.
Was it a family matter that caused Mark to return to Jerusalem ?
Did John Mark have a Roman father, that his surname was 'Marcus' ? And having a Roman father and a Jewish mother, did Mark meet with discrimination, perhaps, from both Jews and Romans ? And was that a discouragement to him ?
But later, both Peter and Paul commend Mark in their epistles and Peter 'adopts' the young man with a (possibly estranged) Roman father :
Marcus, my son : I Peter 5:13. [I do not accept that it is a different Marcus.]
Take Mark and bring him with thee for he is profitable to the ministry. I Timothy 4:11.
So in writing the book of Mark, I can see that Mark was well placed to appreciate and understand the matters of which he wrote and I strongly suspect (myself) that he is the young man who fled.
But I have no proof.