On Star Trek TMP Director's Edition chapter 15 at 50:49, in the scene where Spock has just arrived on the bridge, Decker gets up out of the science officer's chair to make way for Spock. Just before they pass each other, the green-tinted computer screen Decker was sitting in front of reads "MARK" for about 1 second, until Spock passes in front of it. The same screen bears no such moniker before or after that.

Film credits show a number of "Marks" among the various visual effects crew, so I suspect one of them slipped this in. Way to go, Mark!