Periodically I would notice a water mark on the ceiling of my kitchen, directly over my 2nd floor bathroom. The occurrences went from every now and then to every time the bath/shower is used. The shower/tub is a solid fiberglass unit which has a high walls - no tile around the tub. My husband caulked around the faucet and spout but the leak continued. I believe I have found the location of the leak. I stopped the drain (plunger type), poured a cup of water directly on the drain and waited a couple of minutes. I checked the ceiling in the kitchen and it was wet. I then dried the area and caulked, waited 24 hours and tested again (same method) and it still leaked. The house is only 6 years old and there no trap doors to easily access the plumbing. What else should I try or do? Thanks!

Sounds like the seal under the lip of the drain's ruptured and needs to be resealed. Remove the plunger from the drain. The lip of the drain has threads. Back the lip out counterclockwise until there's space under it. Now jam in plumbers putty or silicon jel and tighten it back down. If you use putty the drain can be used immediacy, if jel let it set up and cure. Good luck, Tom