The American Ferret Association has this to say:
Ferrets are known to have their own natural musky scent. This scent is present in all ferrets, whether they’re early spay/neuter ferrets, later alter ferrets, or intact ferrets (not spayed or neutered). Contrary to common belief, however, the natural scent in ferrets has nothing to do with their anal glands. The scent is in fact produced by oils in the skin and is mostly apparent in intact ferrets who are cycling into their reproductive season. Once a ferret is spayed or neutered, most of its odor is eliminated, though a light musky scent will remain. Owners who may find this remaining scent stronger than usual can easily alleviate the problem by replacing the ferret’s bedding (hammock, sleep sacks, etc.) with clean bedding. Frequent bathing is not recommended. This will in fact have the opposite effect to the one desired, since the ferret’s skin will produce more oils to replenish what was lost in the bath.
In addition to their natural musky scent, ferrets, like skunks, are born with the capacity to emit strong odors through their anal (scent) glands. As applied to domestic ferrets, the term “descenting” refers to the surgical removal of these glands. The term is misleading, however, since it seems to imply the removal of natural body scents that, in mild form, are a permanent feature of ferret physiology. These scents can never be totally eliminated, since, in ferrets, they are not produced by the anal glands but by sebaceous secretions of the skin.