Weird A/C leak after driving through heavy rain and the car was half flooded?

So, my A/C system was working great until the car was half flooded while driving during a heavy rainy day, the next day the A/C was not working at all. Then after doing the diagnosis, I found only one DTC (00819) which is related to the high-pressure sensor electric signal. So, I had to change it after the right diagnosis at the garage then they recovered my A/C system and found only 220 grams were found inside and done the vacuuming and charged the A/C system gas at 550 grams and it worked perfectly. But an hour later once I switched off the A/C and the car for about 40 minutes
switched on but there was no gas and just a hissing sound from the vents. I tried to repeat the gas recharge twice and as long as A/C is on it is cold and working no matter how many hours but once I turn it off and turn off the car gas will disappear in minutes.
Unfortunately, until now no one has been able to discover how or where is this huge leak from.

So, do you have any useful advice or ideas on how that accrued and what is the right way to troubleshot if evaporator or expansion valve can be tested?

MK5
FSI 2.0L
ENG:BVZ