Wet Patches On The Concrete

Very strange, this past fall, these patches started appearing on the floor (stained concrete), a week or two after the heating system was turned on.
Initially, the obvious was feared, a leak in the radiant floor heating system. After much investigation which included pressure testing the entire floor heating system (everything was fine, and no leaks found) over a period of time.
This problem became baffling (and still is) as we looked into drainage around the house, for the possibility of build up of ground source water etc. We even dug up some of the grading down to the foundation, to see if drainage was the problem. It was bone dry....

Anyway, over the past couple of months these patches are slowly dissppearing and drying out. We still have not figured out what caused them tho.

If anyone has had a similar problem, please comment as this has everyone baffled.