Hi, it is common for these openings to be left in concrete slabs under tubs and showers for the plumbing drains. It won't hurt anything as long as no support is needed for whatever fixture is over it: tub, shower, etc. Once though little digging animals moles or whatever piled a lot of dirt under my bath tub and I had to remove a wall panel to clean it out. Otherwise it makes it easier for the plumber to align the drain when installing the fixture, as it is not easy (even possible?) to locate the drain pipe perfectly in the concrete slab so it will be in exactly the right place it needs to be. Hope this helps you.