From memory, it wasn't so much that they were fake ventilation ducts, but that they were plumbed in a manner that dropping a grenade in them at the upper level caused them drop out at your feet. Kind of like how flushing your toilet doesn't cause water to flow up from your shower drain.
Apparently some of the German pillboxes/bunkers had a fake ventilation duct. If you put a grenade in, it appeared back at your feet!