Mk 14:51 "a certain young man"
The note makes many words around the fact that this young man is anonymous like other persons - which is unecessary since the reader knows that. The stronger "see also" ref to 16:5 seems to be influenced by the linked sidebar article "the Young Man in Mark" but this essay doesn't give more information but tracks on an "interesting" idea - which presupposes that the traditional interpretation of the man appearing in 16:5 as an angel is wrong (it's not even mentioned).
Notably lacking - which is sad for a Study Bible with an intended evangelical audience - is the information that traditional exegesis tends to identify the young man with the Gospel's author, John Mark.
Mark 14:51Faithlife Study Bible
14:51 a certain young man The youth’s identity is unknown, but it might be Mark himself. This figure appears only in his Gospel.