Odor · no source
An odor you cannot trace
Whether this is a job for the Grand Rapids line depends on where the smell points. If it points at a crawl space, an attic, the deck or a shed, a visit makes sense. If it points at a sealed wall and nothing else, it usually does not.
Narrowing it down
Where is it strongest
At the crawl space hatch, the attic access, the deck steps or the garage points at a reachable space.
Does it change with the furnace
A smell that rises when the heat runs is moving through a duct or a void warm air passes through.
Flies
A cluster at one window or fixture sits close to the source.
What you heard, and when it stopped
Scratching in the ceiling three weeks ago, then quiet, then a smell, locates the room.
Recent work
A capped vent or a sealed soffit can shut an animal in.
The sealed-wall case
For a suspected mouse or rat inside an inaccessible wall cavity, opening the wall is often not the first step. If the carcass cannot be reached without cutting into the structure, the odor will typically lessen as decomposition runs its course.
Finished drywall and plaster are not opened for a small rodent. If that is what you have, ventilation and patience are the realistic answer, and you should not be sold a visit for it.
First, the drains
A dry trap in a basement floor drain or an unused bathroom smells like something died. Run water into every drain and give it a day before calling anyone.
Where the trail usually ends
In a reachable space, which makes it a crawl space or attic recovery. Sometimes outside against a foundation, which is a pickup. Occasionally a deer in the woods behind the house that nobody had noticed. Cost explains why a search is priced differently.