Mystery water intrusion

Hello all. I’ve been battling a leak issue in the rear right side passenger floor well. It rained recently and when I checked under the carpet, the rag I had laid down under the carpet was soaked enough I had to wring it out. All the obvious places have been gone over and sealed up. The weather stripping looks ok as well. The only thing I can think of is that water is getting in from underside? I haven’t been to car wash recently or anything. I’m really getting to the end of my rope, I love my Prius so I’m really desperate to fix this. I’ve even soaked the car with a hose and checked for leaks and the area that was previously wet was still dry. Any help would be really appreciated. My next course of action is to take to my mechanic but I have doubts they will be able to figure it out. Thank you!