I have never smelled human decomp and surely hope I never do. What's interesting is that my brother died in his bed in an apt. building in December (5 years ago) and was found after 2 days. When my mother went to clean out the apt after his funeral a few days later she said she did not detect any odor. Isn't that strange?
My hubby is a funeral director so I asked him about your experience. He said that if you live in an area of the country that has a cold winter and the heat in your brother's apartment was not on very warm it is possible that the cold weather kept his body from decomposing at the normal rate.
Unfortunately because of his job hubby has also had quite a few experiences with badly decomposed bodies and he has said that without a doubt that human decomposition is the worst smell he has ever encountered and it is one that a person could never forget. Also, if a badly decomposed body is in an enclosed area for any period of time that the smell is practically impossible to remove from any porous surfaces in the room.
If anyone has any specific questions I would be happy to ask him if he had any insight!