I was also guessing it is the oil filler.
One house that I heard of - they installed a new natural gas furnace in the spring, and never called the oil company. But the guys who drove the trucks decided to go fill it anyway, and added a full 500 gallons to the filler pipe, but the tank in the basement had been removed. . . . So it kinda flooded the basement a little bit. That was back in the 70's. Nowdays the tank and all it's piping would get removed. Perhaps because it was under a driveway, they left it in place?
Hopefully it is not still connected to a tank. As the tanks could be removed and sold for scrap in the 70's, but now it is more expensive to take one out than the scrap value is worth. . .
Good luck!
Fred.
