Odd smell

Sorry, should read, I don't use the top floor of my three storey house.

I think Karmalady has the right idea. Dry drains.
I don't use the floor of my three storey house. I have to regularly run water, flush the loo otherwise the drains become dry. I become aware of the problem, if I've missed doing this, because of the smell.
The smell actually comes from the shower drain. I put soda crystals in the drain and hot water. The smell clears immediately. I may, as suggested by Karma add a little Zoflora in future. I've not used it before.
Do let us know when you find the cause of your 'smell'. Hope you find a solution.

I am guessing its sewer or water. Plug each for a day then unplug separately to find the culprit.

No flies are a good sign.

For several months we had a horrible smell in one of the bedrooms of our beach house. Eventually we got so sick of it we made a hole in the ceiling to look and there it was- a corpse of a dead possum. Yuk. It was trapped, obviously. Like rats they can squash themselves down and get in but sometimes can’t get out. I was thinking squirrel where you are ?

You don't happen to live in Birmingham, by any chance? The residents there are currently having to deal with swarms of hot-and-cold running rats while the bin men's strike goes on and on.
A relative lives there and he says the damn things are breeding at an staggering rate on account of the mountains of bin bags filled with rotting food.

Maybe a dead mouse somewhere?

Dry drains, run hot water down with a bit of disinfectant, like zoflora. Every drain and every branch. Run the dishwasher too if you have one

Those two rooms both have drainage systems and the hot sun will have dried them

If you smell that smell outside then it will likely be a local tip releasing hydgrogen sulphide gas and that needs a phone call to the environment agency, which takes calls 24/7

Is your electric wiring ok? I did read about someone having a fishy smell in their property that was due to a faulty socket.

Could something have died in a cavity?

Molds and mildew and plumbing problems can happen behind units etc.
Is something leaking behind something? Behind the shower or kitchen units or under the bathroom flooring?
Dry drains smell too.