Thanks! I think this was actually more helpful than you realise! I climbed onto a chair and it certainly seems to be a lot stronger towards the ceiling fan. I can't safely get right up to it (I don't currently own a ladder... Yet another thing for the list sigh), but just based on how it was stronger closer to it than away from it, I think you may be right

I've gotten as close as I physically can to the drains (the shower is too small to bend or sit down in, bathtub would require getting in and lying down, and the sink I can get fairly close to but it's got a small square basin). They all really do mostly just smell like water, a tiny dash of soap, and old people

E: oh and if it is that, that kind of makes a lot of sense now that I think about it. It's been hot today, so it'd be super hot in there, and it would also explain why I can faintly smell it in the toilet room, and also why no other drains or rooms in the house smell bad