I would argue he is looking at the side of the fingerboard, where there are markers. This is what I do as well. Coming from the DB side where I started, any visual cues I used were variations in the wood grain on the side/back of the neck; what I called my "cheat marks".My favorite (living) fretless player, Mark Egan, plays only lined fretless, and notice how he looks at the board ALL THE TIME, and consequently, plays in tune.