Could also be leaky door sills or the ever popular leaking seal on a rear light cluster
Can you answer the following:
1) Was it raining over there?
2) Which corner of the car is lowest when it was parked, or do you park somewhere flat?
3) Saloon, estate or coupe?
4) Is there any dampness around the courtesy light or the ultrasonic alarm sensors?
5) Does your spare wheel well resemble a swimming pool?
6) Is there a sloshing sound when you drive?
7) Can you hear/feel water in any of the doors?

How high's the water level in the coolant reservoir?
<steve_earwig> I think this forum is more about keeping our cars going with minimal outlay than giving our cars more reason to go bang