We have a bad smell in our living room that is much worse in the morning when we open the door, presumably because it has been shut in overnight. The smell never seems to travel to the adjacent hall and there are no other adjoining rooms except two bedrooms above (smell free). We have no chimney and it is a 70's house so not hugely old. The room is wall to wall 80/20 wool carpet that we had fitted when we moved in about 9 months ago.
We did not smell this before the carpets but we were busy painting and glossing which are pretty strong odours and the room was well ventilated for this reason. We had the same carpet and underlay put in our bedroom and we have never had a bad smell here. We have only ever had very minor spills in this room that have always been cleaned up immediately. It does not smell like a decaying animal to me, and I have been good at detecting "presents" from previous cats. I cannot detect the exact source or area by sniffing but it doesn't seem to be our furniture.