I think you are mistaken. There is a materials handling data sheet for Zyklon B, taken from Pressac, Jean-Claude, Auschwitz: Technique and operation of the gas chambers, The Beate Klarsfeld Foundation, New York: 1989, pp. 18-20, posted at page 2 of:Hi. I could be wrong, but I think that the 538c mark is where the gas self-ignites when heated up. I suspect that a small spark would be enough to ignite the gas- for instance a spark from an electric fan engine..(?)
I could be all wrong though, but the mixture of HCN and oxygen is undoubtebly combustable, and perhaps dangerously unstable. If you look closer on the etikette of the Zyklon-B box, ( http://www.zyklon-b.info/produkt/warnstoff.htm ) you'll see that the can is to be kept cool and away from exposure to the sun. also there is a warning to keep it away from open fire.
http://forum.axishistory.com/viewtopic.php?t=47151
At Section I "Danger of explosion" the handling instructions state that there is no danger of explosion in normal applications of Zyklon B, and that the concentrations have to be 750% or more than the normal amount for the danger to arise. The instructions go on to state that:
Prussic acid may not be brought into contact with an open flame, glowing wires, etc. because then it burns up slowly and loses all its effectiveness (carbonic acid, water and nitrogen are formed.) [emphasis mine].