I would suggest your original correspondent didn't quite get the circumstances correct.
confirms that the Rifle Brigade deployment that days was 2 companies in the front line, 2 companies in support at Au Bon Gite and the HQ at U 29 b 8 9. It records the only hostile shelling that day was on U 29 b 68 98. Given that this location is but 100 yards or so from the area that was shelled, I'd suggest that your man was either in battalion HQ or moving to or from it.
MaxD