I've had exactly the same and was due to the A pillar plug - looks like window left open but the water just runs along the seal for a bit then makes a big splash on the seat!Yep I'd agree with that, re-sealed my plugs when the car developed a small drip and perfect since
The plug leaks and the water runs along to top door seal and drips onto the seat (or your knee if your driving)
Could be your problem, worth doing anyway.