Could be under the trim? In the windows? Under the sink? Are you in a warm weather or cold weather climate? Could also be junk trapped in drains.

Cold weather, Philadelphia. Cleaned the disposal well yesterday, don't smell anything coming from powder room sink.

Anyway to tell if it is under trim aside from ripping off our custom made history wood trim from our plaster walls?

Just noticed this green and grey stuff growing on the stepsstucco near the steps outside the kitchen door. We have a good sweep built into the door, but maybemaybe it is smelling coming in under the door?

My fabulous house cleaning guy is coming in an hour. Might have him clean the stucco and leaf matter, and exterior drain cover.

Still skeptical that this is the cause. I don't smell it when I am outside.

I have leaves covering my garden beds in the patio outside the kitchen . My thinking was it is mulch like. Maybe it is rotting leaf smell? Again, it doesn't bother me when I'm out there with my nose to the garden.

There's too much fresh air outside to "smell" mold. If you are smelling it at the door threshold? Inside? There you go. Start there.It's been mild, very wet this year. But not warm enough to dry that entry out. Especially rotting leaves.

That doesn’t appear to be flashed to the stucco correctly. If it’s funneling water into the structure, there’s your issue. Call a good contractor.

Can you remove the interior trim around the door and check the insulation in the wall?
Air infiltration with temperature changes there can create damp spots leading to ongoing mildew growth.

Can you say more about what does not appear to be flashed to the stucco appropriately? So you mean the exterior door frame? It is a custom build wood frame that was described to the stucco and the caulking (or whatever sealant selant it is) has no cracks in it and seems water tight.

I did take a close up of the bottom of the stucco near the ground at the corner where the door trim starts and noticed a slight chip there. I wonder if water has gotten in there?

Notre, the dark lines are not gasgaps. I pressed in there withwith my fingers and it is just dust/ dirt build up. It is all sealed.

Sorry my phone is typing so wacky!

You sure it’s not a dead mouse or critter in the wall? That would smell.

Dead mice, living mice, or just plain old rats do not usually smell like mould. We once had some rodents unknown, likely rats, in a nest in our kitchen wall, how did I know? When I slammed the kitchen drawers they would make little baby rodent noises. In our current kitchen I had a mouse living in the stove, every time the stove was turned on, smelled like hot mouse pee. Blech. I caught that one with a trap. Otherwise, our cats have helped us out in that regard.

If the bottom of the stucco is water tight, that's likely to be part of the problem.

You need the space behind the stucco to allow condensation/moisture to drain. Just like you put weep holes in a brick or stone-clad home, you need the space between the sheathing and the stucco to breath, otherwise moisture gets pushed into the sheathing due to vapor pressure and the sheathing starts to disintegrate (and mold can form).

Sealing the bottom of a stucco wall to make it watertight is bad construction.

https://www.mtidry.com/weep-screed

You also need flashing between the hard surfaces of the steps and stucco.

Got an airborne mold test kit and will run the samples tomorrow. Having a professional come to test it on Monday AM as well. Also ran around my house with a moisture meter. Didn't find anything out of the ordinary, though I get that might not be the most reliable test.

Airborne mold tests will ALWAYS be positive. There are all sorts of mold spores naturally in the air. Indoors and out.

My guess is there is a flashing problem somewhere around your door and water is getting in.

Yes, mold spore tests will always be positive but the question is whether or not the counts are off the charts in a particular part of the house. Samples will also be taken of outdoor air as a comparison to indoor.

How would I know if there is a flashing problem?

I can certainly imagine a flashing problem with the door. We had it installed by a guy who came highly recommended and while working on the job it because apparent he has pretty deep into a progression of Parkinson's disease and had a lot of confusion. It was quite a debacle.

Rent a FLIR. You’ll quickly see the issue. There is no measure to keep moisture away from your stucco there. Traditional stucco has felt paper under it as a moisture barrier. Penetrations or abutments should be flashed. Flashing should be visible in any close up like you show. That moisture barrier is not waterproofing. It is not meant to stand up to full bore water saturation and water directed towards it instead of away from it. https://www.buildingscience.com/documents/information-sheets/common-flashing-details

Thanks Live wire. I believe the mold inspector is coming with an FLIR. We will see what it shows. I can't quite imagine what flashing looks like to the eye. All of the pictures I am seeing of it online show more modern pre-fabbed windows and doors with an aluminum piece. All of our windows and doors are made of wood. I don't see any metal pieces around them. (Sorry this isn't too technical. My learning curve is steep here as you can tell.) I do know that when the laid the marble threshold slab the slanted it away from the house.

mould will show itself eventually. It may breed in hiding but it will eventually show itself in plain view. I believe it grows behind the stucco, lower than the top surface of the stone steps. Breathing mould outdoors would qualify you for canine status. The parts per million is easily identified indoors, but outdoors is nearly humanly impossible when it grows under something. Take a wooden trowel handle or a wooden spoon and start tapping the stucco and brick steps where they join. when you hit the hollow sounding spot you will know where your culprits hide out. The wall already bulges. Look beyond the surface. If its hollow when you knock, you have space for mould visitors.

Kendrah, did you get the issue resolved? I'd love to know what you discovered! I'm having a similar problem at our new place in northern cali. There's a super strong musty smell in one room of the house and I'm wondering if it's a stucco issue... The stucco is not properly flashed, I think. It was built in the 60s.