Puff adders catch prey by remaining motionless until it approaches within striking range. They can stay still for days. Since they are in this position for a long time above ground it should make them easy for predators to find using their sense of smell.
However, researchers from the University of the Witwatersrand discovered that the snakes have evolved an impressive visual and chemical camouflage that makes them virtually impossible to detect. They spent three years radio tracking 30 puff adders to see how effective their scent camouflage is in combination with their natural markings.
When dogs and mongooses, which have no problem in smelling other types of snakes, were released near the puff adders they were almost completely unaware of them, on occasion actually stepping on them and walking over them.
The puff adders, surprisingly, remained motionless, relying on their colouration and lack of obvious smell to protect them. This appears to be an adaption as a result this habit of keeping still, even when threatened.
Puff adders are usually only aggressive when they are on the move, presumably because they have given their presence away at that stage.
I wouldn't recommend testing it out though.