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.

Your duct has came apart and is down or it was never installed. They don't work good with out the return air. You got one good thing going I bet you didn't spend a lot on filters if you use a filter grill.

There is a chance you are actually looking at a very poor attempt to install the return air. I have seen some pretty shabby jobs in my experience, and never am surprised anymore. It is a simple test to put the grille back on, and turn on your fan on the furnace, and then hold a piece of paper up to the grille, and see if there is enough suction to hold the paper to the grille. This would indicate that possibly it is at least partially working as a return air. Other than that, you may need an expert to verify the existence of the return air. The danger is that the furnace may draw air from innapropriate areas like the water heater vent, which could create a negative pressure and draw flue gasses into the duct system. I have seen this on several occasions, and it can be dangerous. Good Luck