Strange...water on floor panel behind passenger seat

I have something really strange that has happened to me...two days ago I noticed a small puddle of water in the back off my Prius on the floor behind the passenger seat. There is absolutely no water anyway else..I've thoroughly searched. I dried it up as much as I can but it seems to continue to get wet - not to the extent it was the first time I saw it, but when I press down with my hand I can hear water in the carpet. I've used a wet vac to suck up as much as I can...but I come back and it is still wet again....Any ideas what could be leaking...again..absolutely nothing else is wet..and nothing could have spilled that would have produced that much water.

 

no rain at all and I park in my garage. I just took out the backseat and managed to lift up the carpet in that area. The foam under the carpet is literally drenched. I am going to let that dry out and try to determine if something spilled or if it is actually a leak. I have my fingers crossed hoping that the kids spilled some water but didn't say anything....