It's the secondary breakdown microbes that cause the odor - NZ warns that absence of odor is not a reliable indicator.   I've found during inspections that people vary widely in ability to smell even far gone cases of AFB.
Case last spring with our state entomologist.  I picked up the odor while still yards away from the hives - and the level of AFB was severe.  About half-dozen beekeepers - none could smell.  After opening hive, a couple notice a faint odor.  Bringing an AFB comb to them and having them sniff, one still couldn't smell.
My biggest problem has been false positives in different areas of the country - where a local floral resource may produce an odor close-enough to raise my concern.   Not quite the same, but first time around, it can be  confusing until my nose/brain learn to distinguish the plant stink from the foulbrood stink.  Amazing how stinky some floral resources can be.  Anyone else notice this?   Maybe generate a list just for kicks?

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