It reminds me of Spock's return in TMP, where logically, Chekov was in the turbolift with him, and if you look closely, when they go back to a wide shot, you can see him just finish sidling up behind Uhura, which means that during Kirk's shocked leap from his chair, and Spock's dramatic stone-faced gaze in close-up, Chekov was off-screen quietly slipping out on to the bridge from behind Spock so as not to disrupt the moment.
