This happened to me a few weeks ago. There was another thread about it then. It has recently started happening again to one of my sensors. I haven’t actually removed it yet, but resetting and repairing had no effect.
Yesterday, two of my motion sensors got stuck on “motion detected”. I got one (not ST) to work by removing the battery and reinserting it. The ST 2015 one (like the one you have) needed me to remove it and rejoin it. Like you it never showed up in the pairing screen, however I later noticed that it has been added with just the name motion sensor. I renamed, re-added it to all my automations and it was good to go.
This morning I walked into my living room my lights didn’t come on. I walked into the kitchen and the lights didn’t come on. I went downstairs and the lights didn’t come on. At the bottom of the stairs I have a button controller that can turn on the lights, I pressed it an nothing happened. I have a door sensor on the closet, I opened the door and the lights didn’t come on.
Ok… …