Maybe all it is is the smell from the bakery somehow getting into the space and my guess would be the exhaust fans from the bathroom somehow doing this. If it did not apper when you were away whatever is cuasing it IMO has something to do with what you do in the house. If it was mold IMO the smell would be worse when you arrived home.

Hmm. There are probably mold mitigation specialists in your area who could see if the mold is actually behind the walls. They might have to cut open a couple of small holes to take samples, so it would likely require patching and painting afterwards, which they may be able to do, too.

But your story reminds me that my high school in the Bronx was about a 1/2 mile down the road from the Stella D'oro bakery and the smell of Italian cookies baking would always greet us on the way into classes. :-)

The bakery is not anywhere near my house (and unless you drive near the bakery, you can't actually smell it across the entire city). Until my Google search, I had no idea this type of yeasty-smell could be mold. I definitely do not want to cut open my walls to find out!

What I would try? Use NEITHER the powder room or laundry for two days.

Now......add each in for use - one at a time. No different than trying to figure out what caused a rash.

Or you check to see if you have mulched too close to the house.

Or you plug your nose.

"Why does my bathroom smell like yeast?

It's either yeast and bacteria growing in a drain, in which case you need to thoroughly clean and sterilize the sink trap(s) and the drain to the tub and/or shower. Also check the plumbing for leaks while you're at it, underneath the sink(s) and tub/shower.........