Floor flooding after rain

It rained a lot one night, but the street I was parked on was not flooded by any means. As I was driving to work, I heard a sloshing noise that sounded like it was from underneath the car. When I got to work, I noticed that my feet were kind of wet. I looked on the floor and the driver’s side flooring was sopping wet. Later that day I was driving home and noticed when on hills that water would flow back to the driver’s side backseat floor (about four inches of water was suddenly on the floor of my car). It is a leased car, so I brought it in to the dealership/service that I bought it from. They are claiming it must have been sitting in water and that is why it flooded, but I am very skeptical of this, and it seems they cannot give me an answer as to how the water got in. Hoping for some answers! Thanks!