Wet carpet mat

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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.