Mine started doing this and the reason was an Android phone I had laying around with an app that scanned for airtags.

Basically it thought it was being used for stalking (close to an unknown device) and started beeping. Turning off the android phone stopped that.

I had a similar problem, the 5 reboots fixed it: https://support.apple.com/en-us/102577

IIRC, this indicates that it's linked to someone else's account, and has not been shared with you. The "beep when moved" feature is to alert people they're being tracked.

For example, I let my mother in law use luggage of mine with an airtag still in it and every time she moved it after the first day or so, it would play a noise.

The battery needs changed.