Art, you're most likly seeing moisture rising up through your slab. Nomally, it just passes through the carpet to the air inside. When you put down the plastic, you provide a cool surface and restricted air flow. If you look under file cabinets you'll probably see the same thing. Another possibility is house depressurization caused by duct leakage of a force air furnace/AC system. Sometimes the wall sole plates are not sealed to the slab allowing moist outside air to be drawn in to a depressurized interior. A little detective work on your part is required locate the source. If you can get to the slab to tape down some poly you should be able to see if there is moisture comming in this way. To do a crude test for depressuriztion, close all exterior doors and windows, turn on the air handler, then crack the an outside door slightly (do this on a day when there is no wind). If the house is depressurized, you'll feel air coming in. If air movement is not obvious, peek through the crack, your eye is very sensitive. It might be very slight, but it doesn't take much.
What to do depends on the cause. For under-slab dampness, your lot should be graded to drain water away from the foundation, including gutter downspouts, etc. If this is all in order, about the only thing you can do inside would be to put down a moisture barrier over the concrete. For depressurization, the ducts need to be properly sealed. This should be accompanied by a combustion safty check if you have any combustion appliances.