I found a puddle in the back floor passenger side?

I live in florida. Average daily temp 90 this is the rainy season I cannot drive but approx. 40 miles a week to therapy. The car is parked outside, and I keep a window cover over the front window. Two weeks ago, I found a puddle in the back floor passenger side. It drained to the front under the seat and under the front mat. The bulk of the water was in the back. I actually thought I might have spilled a gallon of water. I went to my mechanic who is a master with luxury cars and he could not find anything. So I went home and took large bath towels and got it all out and left it to dry in the hot florida sun. Some mildew had already occurred I used a product to get it out. Two weeks have gone by it is back again! A lot less water but it is there ! No lights are on I also took it for a oil change and they saw nothing they put it up on a lift.