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.