This is quite common on Astra J models. Water always flows through the insides of doors on every car. It's impossible to seal the windows against this and doors also suffer from internal condensation. The water is supposed to flow out through drain holes in the door bottoms, but sometimes if the car is parked at a fore/aft angle it doesn't. And, of course, if the drains get blocked with anti corrosion wax, the water cannot drain. It's a simple matter to poke the drainholes through with a wooden skewer or cocktail stick (wooden so you do not damage the paint allowing corrosion to start). I hadn't heard of door bottoms being fitted with plugs or ever seen this, but if yours are then the answer is the one given by the garage.