If it was me, I'd buy off-the-shelf explosive gas detectors (which already commonly exist) and when they went off instead of sounding an alarm I'd release nitrogen or argon gas. Both are inert or inert for all intents and purposes (as with nitrogen).
Inert gases are already used in fire suppression systems. Even if the flammable gas was still flammable when mixed with our almost inert gases, its would still likely slow down the burn and reduces pressure within the ATM/cash point.
However there is likely a realistic limit on how much gas you'd store in the ATM, so they could trick the system into pre-firing, wait a few minutes, and then try again. So you'd likely want to set off the building's alarms to stop such an attack vector (even assuming a 10 minute police response, they likely cannot try the attack twice).
I was thinking along those lines too. A 46" high nitrogen tank holds around 125 cubic feet. As a wild guess, if an ATM contains 5 cubic feet of air then you could completely change the air 25 times with one tank. Does that sound reasonable?
Inert gases are already used in fire suppression systems. Even if the flammable gas was still flammable when mixed with our almost inert gases, its would still likely slow down the burn and reduces pressure within the ATM/cash point.
However there is likely a realistic limit on how much gas you'd store in the ATM, so they could trick the system into pre-firing, wait a few minutes, and then try again. So you'd likely want to set off the building's alarms to stop such an attack vector (even assuming a 10 minute police response, they likely cannot try the attack twice).