Moldy mildewy smell from carpet behind driver's seat

I have a 4 year old Gen 3 and never had this problem before. I pulled up all the floor mats and did a sniff test to locate the mildewy smell I just started smelling when I first get into my car. The carpet behind the driver's seat is where I notice the smell - somewhat faint - but enough to notice when opening up the car. I can't feel any moisture. Does anyone know what is under the floorboard on that side of the car? Any suggestions on what might have leaked? The floor mat above the area doesn't have any odor, so it had to have come from below. Thanks in advance.