Water damage. Possibly a drilled underfloor heating?

Where is the shower?

Vapor barrier inside the inner part of the building? I have - to be honest - only half-heartedly followed these posts in the forum, more as a mod than someone who wants to know. But could it be because of this membrane? Are you sure it belongs there? I hope you get more helpful answers than my questioning born out of ignorance. Regards

The others said, if it were the underfloor heating, you would have a pressure loss. Also, the water would be somewhat warm. The screed cover should also be higher than your drilling (3cm). According to the photo, the water stain also looks quite limited.

Therefore:

Where is the shower?

Are you, , sure that it couldn't have been a one-time event? Or that the shower in the bathroom has a leak? Does the stain dry up, or is it constantly wet?

On a one-time event: rather not. It was soaking wet today, but we already had the first lifting of the laminate last late summer (or back then was it really the laminate expansion due to the heat?). However, I wouldn't know what one-time event that should be. No cleaning bucket has tipped over, wet feet after showering won't be enough for that.....

I have attached a sketch of the bathroom. The shower (tiled) is about 2 meters away. What runs over the tiles from the shower to there, I consider unlikely. Whether something runs into the screed in the shower: could be, I can't judge.

Do you have photos from the shell construction (before the screed, after the installation of the water system)? Do you have a detailed heating/plumbing plan?

Since water is pretty bad for a house, I would strongly recommend a specialist. They should determine the cause and suggest solutions.

I agree with the previous speakers: The screed coverage is usually greater than 4.5 cm. A 3 cm deep drill hole cannot cause any damage (neither to the underfloor heating nor to other pipes on the raw floor). Your floor construction will give us more details.

I don't understand the sentence. Do you mean "it can't be the underfloor heating"? We've been heating the house like this for two years and the bathroom got warm, and we still have 1.5 bar pressure on the heating.

sorry, of course a "not" is missing

again: it is not the underfloor heating if you have 1.5 bar on it. the pressure is max 0.1 bar if the pipe has a leak.

if you look at your pictures, it is definitely the shower. someone must have taken an extensive shower and water ran from the shower into the hallway. do you possibly even have a slight slope towards the hallway? you could try pouring a small amount of water (a glass or something similar) at the shower and possibly catch it again in the hallway with a towel.
since it can't be the underfloor heating, the shower and orientation scream for it, it will probably be that.
of course it could also be a slightly leaky supply line. does it run directly there?

you definitely need a construction dryer because water has run under the screed and you can't get it out with underfloor heating. there are methods without breaking up the floor immediately.

you should IMMEDIATELY report the damage to the insurance and preferably do not state "last year" as the date of discovery - maybe first check what reporting deadline you have according to your terms and conditions.

Vapor barrier inside the interior of the building?

well, a vapor barrier is really not necessary. but what we do have is a waterproof impact sound insulation. the screed must be dry when laying it, so where should moisture accumulate underneath?!