My sister's relatively new house had vents near the ceiling, and vents near the floor. They also had a plastic cover, one cover for every pair of vents. In the summer, they covered the lower vents and warm air from near the ceiling was drawn into the vents. In the winter, they covered the upper vents, and the lower vents worked like the typical cold air return. I never looked at the ducting, but in houses I have looked at, the returns just dump the air into the basement, where it is heated or cooled, depending on the season.
I just had a new system installed, and asked about the cold air returns, which is all I have and are ducted to about a foot from the basement floor. The installer said they could be cut off a bit higher, but he didn't think it would make a lot of difference. When we bought this house the wood floors were covered with wall to wall carpet. When I got around to removing that to refinish the floors, I found a 2 foot by 2 foot hole, which had been filled with plywood. There are a couple of ducts crossing the hole, so I can't duct it to the basement floor as an extra cold air return. I put a nice cast iron grill in the hole for aesthetics, and to keep me from falling through the hole. I'm curious how the hole came to be there. Perhaps the original coal burning furnace was under there, and the house was reducted when they switched to gas.