IMHO, it is the WH's "flag", but it is a crew "call".
Last season, in a chippy, competitve game, I had the QB roll out being hotly pursued by two B players. As WH, my eyes were 100% on the QB and the action after the pass was thrown. Once I cleared that, I looked downfield in the direction I thought the pass went, saw no B players in the area, and looked to LJ for help only to see him chasing the ball out of bounds. I had no idea where the ball went exactly or if there were B players in the immediate area at the time of the pass, no one was giving me any help (we either point to the player(s) in the area or shake the head for no one in area), so no flag was thrown. Watched the video, and we missed.