The question is, how long is the reed switch closed? If you stop using gas while the magnet is over the switch, then it’s drawing a small but measurable current for the whole time until you use some more gas and move the magnet away from the switch, in addition to the current needed to run the emonTH.
The resistor that limits the current is part of the processor, and its value is anywhere between 20 kΩ and 50 kΩ, meaning a current of between 66 µA and 165 µA.
In the case of the electricity meter, we know the sensor draws almost no current (the ‘dark’ current of the photodiode) except for the duration to the light pulse.