I suspect that the crawlspace access beneath the dining room has been covered up with drywall, probably in the basement. I carefully mark the area to be demolished.
After removing the drywall, vapor barrier, and pulling back the insulation, my suspicions were confirmed: the foundation access had been covered up by the flippers.
Here is the crawlspace underneath my dining room, which has been starving for heat during the winter months. Finally, I have a way to solve the HVAC problems and replace the old silcock pipe.
The crawlspace is also full of construction debris that was thrown inside by the flipper.