Is this a really old boiler? The modern boilers are able to turn the gas on and off. Leaving it on will not cause gas to escape
All, new and old, will have a thermocouple that, unless it gets hot from a flame, will not allow the gas to flow. This is the reason you have to hold that button in on your fridge
, it over-rides the thermocouple until the gas-fed flame heats it up.
The old boilers with the standing pilot lights
I know less about.
On this subject, hijack…
If I smell propane gas and the alarm does not go off, then what can i conclude? Gas is not significant to set off alarm, or alarm broke?