Hate to return to the original discussion so quickly, but I figured someone out there may wonder if I ever discovered the cause for this "mysterious wet spot." The answer is YES!
Without going into all the detail, let me just say I had MULTIPLE, undetectable water leaks from swollen copper pipes that I had to replace every copper line with PEX. The mysterious wet spot was a result of one of these "hidden" leaks in the copper line from my hot water heater that ran under my furnace duct. This was the source of the original "wet spot." I believe someone in this thread called the leak a "traveler" or something like that. Whoever you are, you were right.
So, with 98% of all copper replaced with PEX (still need to pipe in the bath/shower), we're in the dry. We had to chunk the soggy carpet padding and berber carpet. Next...new floor covering.
....Okay, guys, you can resume your discussion, now