After living in this house for a number of years, and always complaining about the inconsistent heat temperatures (upstairs cold in winter and hot in summer with a/c on), I took off the cold air return duct covers (which, incidentally, are only located on the first floor) to vacuum as I always do about once a year; but, this time, I took a flash light and a mirror to see where it is going. Lo, and behold, I saw only the tops of ducts crossing this way and that. My question is: If this area is the return duct, shouldn't I be looking INTO a duct rather than on top of ducts? I can see the underside of my hardwood floors.

It is OK to use a wall cavity as a return chase. For one reason or another hvac co. may or maynot use duct in the wall as a return. You must use metal duct for the supply side. Run the systems fan and see if air draws into the vent. It not you have duct problems in the basement. When installing airconditioning adding return air to the second floor area is critical for the cooling to work properly. Call in a hvac co. to look it over