I had the exact same thing happen with my Sonoff motion sensor, replaced the battery several times on the Sonoff, and it would work for a day or so, then just get stuck on “motion detected” and would never clear. It had a ton of false alarms too, and was only a couple months old. I tossed disconnected the battery and gave up.
I recently purchased the Aqara Motion Sensor & Detector - Aqara mentioned by Ardy above. Its been much more reliable, but did disconnect once on me and needed to be manually paired again. I did just upgrade zigbee2mqtt so I assume that has something to do with it.
I’m honestly having second thoughts, there was a warning about Aqara devices on the zigbee2mqtt page not being “fully” zigbee compliant, which leads to issues. The warning is on the zigbee2mqtt link also posted above by Ardy. I would encourage you to read this thread as well before purchasing this - the poster over there had tons of problems with Aqara devices.
Another thread discussed motion sensor options, and suggested building one yourself and flashing ESP or PTVO software. I might go that route for my next one.
The Philips Hue recommended in the post above also seems to be the device that works the best and causes the least amount of trouble from what I’ve read. Its more expensive but the cost of replacing all these cheap unreliable sensors all the time might be worth it.