I've dug up the rest of the concrete slab the other night. Turns out I was almost done and there were only a few more cm to it. Also pried up a piece of the broken edge. It's just a few cm of concrete poured onto the ground in a rectangle.
No sarcophagus, no Ötzi, no secret bunker, not even a pot of gold. Disappointing.
No idea why it was put there. Perhaps something stood on it once.