What to ask, and why
I've had three companies out and nobody found it. What should be different next time?
Hire someone who checks sources in order: drains and traps, damp materials, crawl space air, animals, then the furnace and ducts. Treating the whole house and hoping is what the first three did.
What if it turns out to be a plumbing problem?
Then a plumber is your first call. Odor treatment comes after the pipe is fixed.
How do I avoid paying to treat the wrong thing?
Ask for a written quote tied to what the inspection found, before any treatment starts.
Could it be a dead mouse?
Yes, if the smell is sweet and arrived all at once. Tracing it room by room narrows it to a wall or a corner of the crawl space.
Could a skunk be living under the addition?
If the smell is sharp and sulfurous near a foundation vent, yes. A wildlife operator removes the animal. Odor treatment comes after it is gone.