Water is collecting in the right rear floor-board. The first time it happened I thought it had

come from a water bottle I found under the seat. I checked it after a big rain back then and no water was found, so I was sure that was it. Now a month later, cleaning out the car, and I find that the floor-board is wet again. I looked hard and there is no water bottle to blame it on this time. There is no evidence of any water running down from anywhere. I never notice any coming in when it's raining. Where is it coming from? I haven't a clue.

I'm thinking it is coming from under the car, but I can't find the culprit. Any suggestions anyone? I don't have much hair left.