In-ground winterized leaks like the last winter

[Roberto]

Good morning,
My 3.5x7x1.5m inground pool has a really strange leak.
Throughout the summer it seemed to be leaking, but the leak was very minimal.
I live in Hungary, it always freezes in winter, so when I close the pool I remove the pump, empty the filter, lower the water level under the skimmer and the water return holes.
Well, as I did these jobs the water level began to drop drastically.
Last year (in winter) he did the same thing too. The level drops quickly, even under the pool lights and stops about 50cm above the ground.
Could it be the suction pipe located a few cm from the bottom?
But it's strange because the technical part is underground right behind the skimmer and the pool bottom suction pipe. It is always humid but not flooded, there are no puddles of water.
My pool is not equipped with a hydrostatic valve. There is nothing at the bottom of the pool. Could it be elsewhere?
This summer, since last year in winter I had the same problem, I tried to look for leaks but I couldn't.
Then, as I said, I left it alone as the water level dropped but only slightly.
Could it be a leak caused by the groundwater?
Any ideas, advice or criticism are welcome

(Sorry for my English)