Why is there a hole in my slab?

We had a leaky pipe burst over the weekend. It seems that it had been leaking (slowly) for some time, so there’s some water damage to the wall where it was located. In order to clean it up, I removed a cabinet on the opposite side of the wall from where we repaired the leak and found an unusual hole in the slab.

This cabinet is in a ground-floor powder room. The leak was in one of the exposed pipes in the wall on the left. The pipes running horizontally along the back wall are the result of a reroute of hot and cold lines due to two different leaks under the slab.

Some background: the house is in Southern California, constructed in the early 2000s.

The thing I’m curious about is why there’s a hole in the slab, exposing the dirt below? My hypothesis is that the original owner had the option, while the house was under construction, to have a shower installed here. If that’s correct, then I would expect to find a drain underneath that dirt.

If we wished to cover that dirt, what would be the best way to go about it? Can we did out some of that dirt, mix some concrete, and fill it?