2002 BMW 745i Question: water on the floorboard of the passenger side

Use a white paper towel to see if the fluid has a green or orange color if it does have color its more than likely your heater core is leaking, If the heater core is leaking your windows will stay fogged up. If your carpet dries out after a few days the window was slightly open during the rain storm. Heater core replacement is a very expensive job because its located behind the dash board.

Adding to what hemi said, a leaking heater core will smell like coolant inside the car (passenger compartment). Coolant has a very distinct smell, sometimes described as a sweet smell but not everyone notices the smell.

Like you said, many leaves gathered at the base of your windshield can lead to a water leak on the passenger floor. If your BMW has a sunroof, those drains can clog too. If it's possible, park it in the garage with the windows cracked to dry out. Once it's dry, you can test for leaks with a garden hose, weather permitting

Water may be seeping through your sun roof and channeling down to the floor through the doors. It has happened to my car.

drainage tube is clogged from ac unit (condensation drain)

Just had it fixed on my 2003 745li it seems to be a real big problem with these cars its your blower that is cracked under your hood that is the problem BMW fixed mine less than 5months ago it is cover under your insurance all I had to pay was my deductable if you let it go it will short out all your sensor underneth your carpet sad thing is mine is doing it again so I will have to put it back in the shop.