In the past, I made several battery powered modules with a reed contact to detect the opening of a door or window. These work well, but are not very reliable in the long run. So, I got the idea to replace the reed contacts with a Hall effect sensor, which are cheap nowadays, and available in abundance, and even smaller.
The most well-known Hall effect sensor in the Arduino world is the A3144 from Allegro, since it is often used in Arduino modules, like the KY-003. Allegro specifies for the A3144 a power supply voltage of 4.5 to 24 Volt, and a supply current of 4.4 to 9.0mA. This is not very useful for battery powered systems.
My search for a low power Hall effect sensor lead to the SL353LT from Honeywell: it consumes on average 1.8µA and works from 2.2V. Far better for battery powered applications!
There are probably others too, but this one is cheap and available.
I ordered a few of these - later I hope to experiment to find out how they perform. Follow my progress at my page about the discrete component SL353LT.