"It is my understanding that there is supposed to be some sort of water proofing painted on the dry wall"

Tile on drywall in a wet area is not an acceptable insulation.

There is a type of dry wall used in baths, isn't there? I think it is called _?_board.
What sort of other barrier is needed? Can you tile directly to this board or do you need something else on it? Thanks for your thoughts.

Green board?

It is no longer allowed under most codes, and was never accepted by the tile industry.

There are some very new products available, maybe they used one of them.

I (ad my go-to tile guy) prefer solid membranes and careful installation to make sure there are folds instead of cuts to form the material (seamless is still best).

We do not use the liquid ones in a can.

House was built less than 2 years ago. So how would we determine how tile was installed? Plan was to remove tiles here and there to see what we could see. In your opinion could it be a grout issue? Pinhole or cracks? When the water gets to flowing it is a steady stream of water that leaks to floor below. Entire tub has new silicone caulk everywhere. Still leaks.

Grout is 'waterproof' only to the extet it is not damaged by water.

like any Portland cement based product it wicks water very well.

That is why a barrier is required behind tile with a grouted joints to stop the water from moving further into the structure.

If by "tub/shower combo"

You mean a conventional bathtub with a shower installed also, look hard at the tile-tub joint.
If it is not installed correctly and caulked it is a very common source of water escaping.

The overflow from the tub is another spot.

Even the faucets can allow water to penetrate into the wall if they are not installed correctly.

The entire tub/tile connection has been recaulked. Carefully and extensively. Water is not getting out via the tub/tile connection. All faucets and spouts and such have been thoroughly checked from behind and they are dry as a bone.

Your thoughts are greatly appreciated!

Just an occasional lurker to this forum and not a plumbing expert by any stretch of the imagination. Trmatty, sounds as though your water leak has been corrected. I just wanted to mention the name of the product that was used in our building projects for around tubs, showers/wet areas in bathrooms. This is Durarock, which the contractors used instead of drywall in those areas. Stuff looks like concrete attached to a mesh (in my very unprofessional observation). You should be able to tell what has been used in those areas when you remove some of the tiles. I'm sure there are other waterproof products as well, but I'm not familiar with those.

This is similar to some tenant experiences we've had. so consider these as possible sources of your problem:
a) unlevel tub, such that it's tilted toward the room
b) too much water splashing those front edges (even if it is level)
c) any water in those areas may be running to the floor or behind the tile at the front edge, then finding it's way to the ceiling below. If everything is not sealed, including outside front tub edge and tub/floor joint, this might be your problem. This is especially true if there's an oversized grout area instead of your tile closely following the contour of the tub at the edges. That happens to be right where the tub's tile flange ends, leaving a gap behind the wall material that can be a problem.

For what it's worth, when I first encountered this with some tenants, I found it hard to believe that so much water could be getting down there by the route I described, but this is exactly what was happening. After a fiberglass enclosure was installed over the old tile, I still ended up glueing some of those splash guards onto the front corners, as well as sealing everything on the outside edges down the floor.

By bet is not that it's bad grout, per se, but that there's a big vacancy behind the grout near those front edges. Before tiling, I like to close that gap with thinset and mesh tape (I'm a bit paranoid about openings like that). Also, I wonder if the inside corners (and any other greenboard joints) were taped, or just left open. Although this case doesn't seem to have water coming from the inside corners, it certainly can happen when those corners are not taped closed before tiling.

You have done extensive troubleshooting, and pretty much eliminated all possible sources of the leak. I do not think it is plumbing related, based on your all the investigative demolition. (How much has that cost?) This is a bad tile job. The next step is to tear it all out. Right down to the studs.

Examine the tub installation: Is it level? Is the rim supported to the walls around the perimeter on all sides with solid 2x4? Is the bottom supported continuously to the subfloor?

Now look at the tub tiling flange: Is it flush with the wall so tile can go over it? Is it tall enough to contain the water?

You may need to have a new tub installed, or the same one reinstalled properly.

Next, cement-based surfacing for the walls: Hardiboard, Asbestos-cement board, or mortar bed, is industry standard for wet areas. Do not use gypsum paneling, even if it will be coated with waterproof membrane.

Install waterproof membrane, like Kerdi, set into a layer of thinset. Make sure the membrane covers the tiling flange.

Install tile over the membrane, after it is completely dry. Wait a few days before grouting to allow all the mortar to cure completely. Waterproof membranes really slow down the thinset from curing. UN-modified thinset is required.

I had these problems in the past, and even though the grout looked good, that was where the leak was. Your experiment where you covered it all and the leak stopped bears this out. Narrow down your range by selectively covering different walls of the shower. Once you identify the right one, route out all the existing grout and regrout the whole wall. Use the right kind of grout for the size of your joints, and make sure you follow the drying directions to the letter.

The thing should be waterproof without tile or grout. I've seen many bad tile showers, usually it's many things done wrong, not just one mistake. The bad jobs start out wrong from the very start. With the right waterproofing system, you can take a shower before the tile or grout is installed, and see that nothing leaks. The tile and grout are not part of the waterproofing system.

Trying to make a leaky shower waterproof with new grout is like putting a band-aid over a tumor.

A pan liner for a tile shower? Why bother? Just use the right kind of grout.

If anyone feels like a new grout job corrected their leaky shower, good for you. Peace of mind is worth the price paid, especially if it also buys some time. I hope the new grout holds out the water forever. But I wouldn't count on it!

I have never been one to argue that one should not do things right, but there is a cost/benefit equation to everything. If the OP does not have the skills to execute what you've suggested, they may also not have the money.