I discovered an intermittent leak on the passenger side front floor panel.
when it rains, the floor mat gets soaked at the bottom of the mat near the
seat, not at the top of the mat. When I've tried to locate the leak, the
only area that appears wet is the middle of the floor mat. The door has been
completely shut.

This only seems to happen when the front end of the car is on a decline.
Has anyone experienced this before?