What it says is they don't actually know what the dogs smell. They can't reproduce it. They know it includes putrescine and cadaverine, but not what else is there which allows the dog to identify the "bouquet of death". There is obviously something else, or the dogs would be alerting to all decaying organic material and human saliva. The point is that they don't alert to these scents, not that they do.
No dog is ever "off duty" at any time. They spend their lives sniffing every inch of their environment, they mark their territory and they know who has been in their territory, they introduce themselves to other dogs by sniffing and being sniffed, they introduce themselves to humans by sniffing.
They are trained to switch on and to switch off to scents which their handler has trained them to respond to as a game. They are playing a game and they know when their handler wants them to play and when their handler doesn't want them to play.
They are not calibrated scientific instruments ... which is why their alerts can only be taken as indications ... the clincher has to be corroborating forensic evidence.