My wife and I had a mysterious musty smell in our master bathroom shower drain for a few months. We tried various remedies - snaking the drain (removed a small wad of hair), over the counter drain cleaners, enzymes, baking soda & vinegar, none of which helped. Then a guest came to visit and used the shower in the guest bathroom on the other side of the house. The smell disappeared. I didn't immediately make the connection, but eventually realized that it might be related.
A year later the smell returned. Having realized what helped the previous time, I ran the guest shower (which we rarely run) on hot water for about 5 minutes. The smell improved after the first day, so I ran the guest shower again, and after the second or third time, it was completely gone again. It has not yet returned after a week, and I suspect it won't for about another year (unless we have some more guests come stay with us in which case it will probably take longer to return).
I'm curious why running the guest shower improves the smell in the master shower? First, I would think that smells shouldn't come up from the drain if there's a proper "S" pipe in there, right? Second, I'd think that the smell would affect either both drains or only the guest shower drain, since that's what eventually cleared it. But the smell never appeared in the guest bathroom as far as I can tell. (While we don't use the guest shower often, we do use the toilet in the same room and would likely have noticed the smell.) What's going on here?
I should mention that the guest bathroom is off the kitchen, and directly behind the wall with the shower head is a small closet where our garbage is. There is a trap door in that closet that you can open to access the shower's plumbing, and I noticed the smell coming from the trap door the last time it happened. But again, after running the shower it went away and hasn't returned. Not sure if that's a clue, but thought I'd mention it just in case.