The garden pond is overflowing into the unknown. How do I find out where it goes?

When it rains, water flows in there, but it never gets fuller than it is - that means there is an overflow or a leak somewhere.

The "pond" is filled directly by the visible downpipe and despite continuous rain it always has a maximum level and does not overflow?

In picture 3 there is an overflow visible at the first chamber.
Maybe it flows over the 3 chambers and then back to the virtually interrupted downpipe at the facade.

Or where exactly is the maximum fill level?

Over the years, the pond was filled through the one vertical pipe on the left side. For about 1 year, we have also directed the second half of the roof there (the pipe coming from the right). The 3 chambers are out of order (they have leaks inward), there is no visible outlet – and: yes, it always stays at the same level, or rather, it never goes beyond it.

and: yes, it always stays at the same level,

Then take a pump and suck off a few hundred liters, then you will discover the "overflow." Or should I come by? :D

suck off a few hundred liters

I have already done that, but stopped before complete emptying (because it became too cumbersome)

Or should I come by? :D

But bring beer in case it takes longer.

I already did that, but stopped before the complete emptying (because it became too cumbersome)

You speak in riddles. A (submersible) pump does not care whether it runs for 0.5 hours or 5 hours.

But you should only look at the overflow; the "pond" does not have to be empty for that. :rolleyes:
If the water level, for example, is always 14 cm below the edge, then there must be something at that height that limits the fill level!