Not all rooms and not all of the time.
The kitchen was fine, as was our bedroom. It was only when I noticed that the smell was particularly strong while sitting watching TV that I had an idea.
Perhaps the thing that was smelling was outside? Perhaps it was down in the undergrowth in the small sunken area by the sitting room window? (Our house is built on a slope and has 2½ storeys.)
Half way through the afternoon today, I caught that whiff of decay again and went out to investigate.
Yes the offending object was exactly where I'd deduced it must be and it turned out to be the corpse of a small bird.
The house smells much better now. *g*