Well, after I posted these pictures, I noticed what appears to be a CRACK right under the drainage hole. I had not seen that before! Could this be the culprit? If so, what in the world do we do about this?

drainage hole to where? You want the perimeter sealed. Your photos seem to show a clear path between the tub and your surround right behind the metal channel of the door frame. (where then staining is) That should be sealed.

The only "drain holes" I can thin of would be to allow water that runs down the dorrs and into the slider channels to drain back into the tub. The ends of the channels should be sealed.

I don't think water will go through the hairline crack in the tub.

I agree. What I see appears as if water is getting behind the wall channel as sdello describes. If you seal between the wall and the inside of the channel, that should solve the problem. That bottom area in the picture definitely needs some sealing.

As for the crack, I agree that water probably isn't going through the hairline crack. The next time you use the shower, take note if the crack seems to open a bit. If not, you could use some enamel paint on a paper towel and rub it onto the crack to hide it. If it is a fiberglass shell, you could also use a small amount of fiberglass resin on it. If the crack does appear to separate, you'll have to do a bit more work. If you have a boating store nearby, such as boaters world, they'll have everything you'll need to fix it and touch it up.

Although the materials used to fix something like that aren't necessarily things that most people use every day, I think it is well within the realm of a do-it-yourselfer to fix.

This post was also in the plumbing forum. One forum, please!

When my husband installed a shower last year, we got a very similar water stain. We have a different shower design, our door meets up with a curved area of shower enclosure, not the wall- but basically he didn't caulk along the seam where the door frame transitioned into the wall of the shower enclosure. (I hope that makes sense!) He assumed it would be impossible for water to get in there, the direction of the shower spray doesn't even hit that area. After caulking it though, no new water stains.

Unless it's against forum policy rules, I was always taught the more opinions the better! Hitting up multiple forums that are geared for the op's question is a good idea in my book! If it's against the rules, I stand corrected.

Thanks for all the great suggestions!

Update: We removed the board from the wall and did a couple of experiments once we could see better what we were dealing with. There didn't appear to be any water damage behind the tile on the wall, just to the drywall and wood right near where the shower meets the wall. First, we ran water (without someone standing in the shower) and didn't see any water seeping out. Second, someone stood in the shower. When water splashed off the person and ran near the corner we could see a tiny trickle of water between the metal and the wall. Once everything is dry again we will re-caulk that area (DH is good at caulking, thank goodness!). Once that is dry we can re-test. Hopefully, that will correct the problem. If not, we will call someone out to fix the hairline crack, but we are fairly sure (as other posters have stated) that it is not the root of the problem.

I posted this in this forum and the plumbing forum because it wasn't obvious to us if this was, indeed, a small repair (which it appears to be) or if it could be an issue with plumbing behind the wall that we couldn't see. I have been a member of these forums for several years (particularly kitchen and home decorating) and see people post questions in multiple forums quite a bit. Especially if the issue is not straight forward.

OK is you still have a problem see if the rubber thing on bottom of shower door(may need mirror & flashlight to see it) is going all the way across under the shower door. It is in a track & simply pulls out. If it is sticking out at the end that isn't leaking push it toward end that is leaking until it is tight at that end & no water leaks out. Whenever you are recaulking don't just go over the ugly mess. take a dull knife like a paring knife & get as much of old stuff out & scrape down to tub or tile & be sure there is no gunk left, wash it off & let dry & then recaulk. People that don't do it this way may have it look better for a very short time but then it just has to be done all over again. Another thing to think about is where the shower is aimed, if it is too close to shower door it will leak out no matter what you do. A needle or straight pin will clean a lot of gunk out of holes in shower-head & then soak in vinegar few hours. Good Luck!