Locating an unknown leak inside of a wall can prove to be a very challenging undertaking, but there are a number of things we can do to find it before we resort to tearing walls out.
We must keep in mind that even though the water is visible on the pipe below, that does not guarantee that the leak is originating from a drain pipe. The water could be leaking from a point above a horizontal drain pipe, then cling to the pipes by surface tension and run a considerable distance until it finds a low spot and drips off or reaches a vertical pipe and runs down the outside wall of that pipe.I once spent three days looking for a plumbing leak only to finally find out it was a roof leak at the base of a Citizens Band Radio Antenna mast on the roof, which was allowing rain water to drip down into the attic, across a truss and dripping down the plumbing vent chase leaving the impression that there was a leak on the vent stack.
Locating leaks is one of the tasks of a plumber which really cannot be taught in the classroom because each situation is a bit different. Fortunately we do learn from experience, so allow me to share some of that experience with you and perhaps we can find your leak without tearing the walls out.
The first step in locating a leak is to have a thorough understanding of how all the piping is connected inside the walls. From this we can surmise some of the potential sources of the leak.
You stated that your tub and shower are tiled and there is no mention of a service access panel therefore the waste and overflow kit should be a glue in type as seen in the above illustration.
In a glue in style kit the TEE, Shoe, and Overflow body are cast PVC parts which are then interconnected by 1-1/2" PVC pipe cut to fit and it is all glued together. When completed there are no mechanical joints which could create a leak.
If you will examine the TUB SHOE you will see that it has a rubber gasket that fits on the top of the shoe and is pulled up tight against the underside of the tub when the drain basket is screwed in. Either plumbers putty or silicone is applied under the flange of the tub basket to seal it to the tub. This is perhaps the most common failure point, especially in fiberglass tubs which tend to flex slightly from the weight of people standing in the tub while showering. I would recommend using a Basket Wrench to first remove the tub basket. With the basket out you should be able to press downward slightly on the tub shoe and insert the tip of a silicone sealer tube spout between the top of the rubber gasket and bottom of the tub and apply a good film of silicone on the gasket. You can then apply silicone or plumbers putty on the underside of the tub shoe flange and screw the drain basket back in tightly. (If you use silicone you must wait 24hrs for the silicone to set before running water, if you use plumbers putty you can run water immediately after tightening the drain basket in place.)
It is possible for the gasket behind the overflow to leak if you fill the tub up to the overflow, but it is very highly unlikely that you will have a leak there if just using the shower.
You stated that you have removed some sheetrock from below, so this would be a good time to examine the Waste & Overflow, P-trap and drain line connections to be sure they are not leaking. They should all be glue joints so the likelihood of a leak is very small.
Carefully feel the underside of the tub spout, especially near the tip where the water comes out. Quite often the chrome finish will corrode leaving a very rough surface on the underside and eroding the rear part of the nozzle away. Once this happens water will cling to the underside of the spout by surface tension, run down the underside and eventually leak inside the wall. The solution is to first replace the spout, then apply a bead of silicone caulking to seal the spout at the wall however, if your spout shower diverter is built into the spout, do not seal the spout to the wall. Diverter spouts have a weep hole on the underside so that if the diverter valve leaks internally the water inside the spout body will drip our the weep hole and down on the tub apron rather than going in the wall.
Remove the faucett handle and Trim Escutcheons, then using a flashlight, look in the hole to see if you can see any evidence of leaking inside. Pay particular attention to the packing nuts at the base of the faucett handle stem shaft enters the stem unit. Also check where the faucett stem assembly connects to the mixer body. If you see evidence of leaks there you can use a plumbers socket to reach in the hole and tighten the stem assembly or the packing nut. (You can buy a complete set of plumbers sockets in any hardware store for about $10)
Next check the shower arm connection. First slide the shower arm trim escutcheon out on the ahower arm pipe away from the wall, then examine the threaded connection where the shower arm connects to the shower riser. If you see any sign of leak here unscrew the shower arm pipe and clean the pipe threads with a small wire brush. (a copper fitting ID brush such as used when soldering is excellant. They can be purchased at any hardware for about $1).
Now apply 3 full wraps of RED TRIPLE DENSITY teflon tape or pipe dope to the threads and re-install the shower arm pipe.
Check all caulking joints in the tub/shower enclosure. If they appear loose the best solution is to cut out the existing caulking and install new caulking.
At this point you will most likely have found and resolved the problem. If not, and if it is necessary to open the walls up, it is best to try to come in from the backside of the wall from another room. In this manner you will only have a simple sheetrock repair instead of replacing the tile.
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