Mike
John and Trevor have covered this well, but I am intrigued by the source of this apparent odour. If it's real, it may be more noticeable first thing in the morning when the rooms door is first opened. It's also not too uncommon for the source to be from mould that could be present on, in under or near the carpet, so check doorways and windows, corners, behind floor length curtains and check to see if central heating pipes disappear into the wall
I've come across this where a curtain pelmet or perhaps a nail for a wall mounted picture has penetrated a hidden water pipe, Drips inside the wall have meant mould growth which is then highlighted by the cleaning.
One problem I had many years ago after cleaning was a musty smell which wasn't previously obvious (perhaps because it was a trashed carpet/room). After the normal analytical approach, I discovered that the teenage children walked around bare footed. I detected the slight hint of sweaty feet in front of the sofa. And before you ask, the answer is yes, Madam and I were on our knees sniffing the carpet together
Safe and happy sniffing
Ken