I am a landlord. I deal with bathroom leaks regularly.
Your leak is not in a water supply line. It is from either a bad drain, or water leaking from the shower stall.
From experience, well over 95% of those bathroom leaks above are due to water leaking from the shower stall. Not a pipe/plumbing issue, but the integrity of the shower stall itself. This is also true with tub/shower combinations.
It only takes about 1/4 cup of water spilled out of a shower to produce a large stain on the ceiling below.
Things to check:
1. Is the caulking in solid clean condition?
2. How is the tile the grout?
3. Is the shower curtain in good condition? And does it cover to opening well?
4 Do you have a good bathmat? This sounds trivial, but there is enought water dripping off the average person to stain the ceiling below.
5. Is there any water on the floor after showering? You have to feel for the water as often it runs down a surface and is not very visible.
If all of the above issues are taken care of, I would suggest a water test. Pour several buckets of water down the drain without getting the tiles, grout, caulking or floor wet. Make sure you pour as much down as when you would shower. I doubt you will have a leak. But if so, the problem is in the drain. If not, use the shower again. If a leak still occurs, keep checking the list above as you have missed something from one of those 5 check points.