Lansing area
You can smell it. You cannot find it.
Some of these are a job for the Lansing removal line and some are not, and the difference is whether the source could be somewhere a person can reach.
A smell that is worst near the attic hatch, at the crawl space door, in the garage, or by the deck steps points at a space the operator can get into, and that is worth a visit. A faint smell in one interior wall with nothing else going on usually points at a mouse, and that case is covered further down.
What narrows it down
Where it is strongest, first. Then whether it changes with the furnace running, which points at a duct or a void warm air moves through. Then flies: a cluster at one window or light fitting sits close to the body. Then anything you heard in the weeks before, and where. Then any recent work that could have shut an animal in, a capped vent or a sealed soffit.
Put those together and most smells resolve to an attic, a crawl space, a soffit, a chimney or the space under a deck. All of those are reachable, and all of those are a normal job.
The wall-cavity 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.
Operators do not cut finished drywall, plaster or ceilings to retrieve a small rodent. If that is what you have, the realistic answer is ventilation and time, and you should not be sold a visit for it.
Rule out the drains first
A dry trap in a floor drain or an unused bathroom smells like something died. Run water into every drain in the house and wait a day before paying anyone.
Where the trail usually ends
In a reachable space, which makes it a recovery job from the attic, crawl space or under the deck. Sometimes outside against the foundation, which is a pickup. Occasionally a deer in the tree line nobody had seen. Cost covers why a search prices differently from a lift-and-go.