What to ask, and why
I've had three companies out and nobody found it. What should be different next time?
Look for someone who works through sources in order: drains and traps, damp materials, crawl space air, animals, and the HVAC, rather than treating the whole house and hoping.
What if it's a plumbing problem?
Then a plumber is the right first call. Odor treatment should come after the cause is fixed.
How do I avoid paying to treat something that isn't the source?
Ask for a written quote tied to what the inspection found, before any treatment.
Could it be a dead mouse?
Yes, especially if the smell is sweet and sudden. Tracing can narrow it to a wall or crawl space area.
Is a skunk under the addition possible?
If the smell is sharp and sulfurous near a foundation vent, yes. A wildlife operator handles the animal; odor treatment comes after it's gone.