From our rules interpreter:
Play: Team A trails by 2 points with 10 seconds remaining in the 4th quarter and the clock running. Team A has no timeouts left and they face a 4th down on B's 15-yard line. At the end of the 3rd down play, A63 runs up to an official showing him that his helmet has come off during the previous play. The official believes that the player has deliberately taken his helmet off to take advantage of an official's timeout to get the field goal unit onto the field.
Ruling: Rule 3-5-10d mandates that an official’s time out must be taken to allow the helmet-less player to leave the game for one play. Penalize Team A 15-yards for Unsportsmanlike Conduct under Rule 9-5-1f, leaving the field between downs to gain an advantage. That’s exactly what the helmet-less player is trying to do….by removing his helmet, he has created a reason to stop the clock and leave the field between downs to gain an advantage for his team.
Following penalty administration, invoke Rule 3-4-6 and start the clock with the ready-for-play. If time expires, the period will not be extended for an untimed down. Rule 3-3-4b1.