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It's either a reed switch or a hall effect sensor. I can't remember the last time I saw a naked glass encapsulated magnetic reed switch except in a magnetic security contact. The reed switch would definitely be the low power(actually no power)option, but it would be easy enough to temporarily power a Hall effect sensor up a few times a second and look for output during sleep mode... of course, a reed switch could go straight to an interrupt with maybe a small cap/resistor filter to minimize bounce. The funny part is, I'd almost bet the hall effect sensor would be cheaper and easier to source these days.
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