Just FYI (couldn’t find reference to this elsewhere): the battery failed on my Samsung motion detector - battery went from 69% last recorded, to not working (measured 2.2V). Replaced with a GP battery - I know! - not the right size! Well, side by side they look identical.
Anyway, couldn’t get the sensor’s LED to light, or the device to pair. Tried prying out the battery contacts on the motion sensor a little and that solved it - they’d been squashed back behind the moulding and weren’t making contact.
PS In the meantime I’ve ordered a couple of the Xiaomi motion sensors (big fan of their stuff) - doubtless I’ll be back here for help when they eventually arrive