Home/Odor
A smell and nothing to see is the most common way one of these starts. It is a solvable problem, and it runs in a particular order.
Which room, when it started, whether it changes through the day.
Most of that happens without touching the house. It is why a description over the phone is worth giving properly: half the search is reasoning about the building, and that part can start before anyone arrives.
A smell that appeared three days ago and is getting stronger is an animal that died recently, and it will keep climbing for a while yet. A smell that has been constant for three weeks is past its peak and is now coming from saturated material as much as from the carcass. Those are different jobs at the point of removal, and the second one is why taking the animal out sometimes does not finish it.
Whatever it was lying on comes away with it. Insulation, dust, cardboard, soil under a deck board. Left in place, that material carries on producing the smell for weeks after the animal is gone, and it is the usual reason a house still smells after someone has already been out and removed something.
Where the search ends inside a structure, what can and cannot be reached sets out the rest. Canton dead animal removal covers this search as well as the removal that follows it.
It depends on the size of the animal and how dry the space is, and the range runs from about a fortnight to a couple of months. Nobody can give you a date. What can be said is that it climbs before it falls, so the early days are not a guide to how bad it gets.
They work on the air in the room. The animal is not in the air in the room. You can make a house liveable for an evening that way, which is sometimes worth doing, but the smell comes back on the same schedule it would have anyway.
Warmth and moving air. A structure that heats up through the day pushes more out of a cavity, and a heating or cooling system running redistributes it. Rising and falling across the day is ordinary behaviour for one of these, and it is not a sign the problem is sorting itself out.
It might be, where the animal was small and the space around it stays dry. Fading also happens when a carcass dries out while the material underneath it stays saturated, and that returns in the first warm spell. If it comes back, it did not finish.
That is an outcome worth naming in advance. Some cavities give nothing away, and after a reasonable search the honest answer is a probable location rather than a certain one. You should know which of those you have been given.