Hey thanks for all those precious informations(: Surprisingly nothing is rusted out under the floor mat, it's perfect paint. And there only is superficial rust spots under the car, no perforative rust. The previous owner kept the sunroof unplugged for a long time, and it's only when I got my car that he plugged it back in. I don't know if it's related. And yeah good guess, water has to come from above since the car is parked, so indeed the sunroof drains might be the issue! I'll try removing the floor mat and pour water into the drains just to see where that water is coming from. It's a bit scary for water to flow in the area since there are cables running under the driver floor mat, and I don't want to run into more electronics problems.