Well, it took some time, but I may have found the answer. As we went through the basement inspection, I noticed some light coming through the subfloor (pic 1). It was openings cut for air return - the return plenum had not been closed up to complete installation! Then, found three fire code gypsum panels screwed to the ceiling …hmmm WTF? Said "I’d really like to know what’s behind that. The realtor says “Tear it down.”! I say "No, can’t do that, but the client says “I will”, and pulls one down (pic 2). Well, well - there’s some cuts through the subfloor and I can see all the way up to the 2nd floor. So I go up there and find a return air grill, shine a light down and, yes it’s one of the holes. Then I check the return plenum again, and it is open on this end too (pic 3). So the guy had never finished enclosing to bring air back from another room. Then I check another room upstairs and it too has a return grill but no path through the wall to basement. This is above the area where the joists had been cut. He must have been trying to get an opening (pic 4). see patched OSB…and just gave up. Then put up a support wall to “fix” the cut joist mistake. The End.