Mystery bluetooth device?

No battery powered BT mouse or keyboard lurking in the next room or the man drawer?

Did you try booting into Safe Mode? (Shift held down while booting).

Then reboot as normal.

That can clear various caches.

The relevant .plist is possibly corrupted. (It can be proved by trying another User Account but it is a faff.)

Go to your finder then in Go, select Go to Folder, then paste the following in the box

~/Library/Preferences/ByHost

Find the plist that looks like "com.apple.Bluetooth.9CC5D1D5-2FDD-5962-A570-2B1DA84B4B7F.plist"

Drag it to the desktop so it is no longer in the ByHost folder

LOG OUT of your user account

Log Back In

It should create a new plist to replace the above one once you connect to a new BT device.

Try your problem again.

Let me know if it is fixed.

Delete the plist in the desktop once you are happy.

ok go through the process of creating a new User Account with Admin access.

Does it have the same problem?

No - then it is some user file but not the above so you need to keep looking

Yes - need to look at a system wide problem or situation.

I had one of those "interloper" bluetooth devices showing up on my iMac. I opened the Bluetooth Pref window clicked on the device name and then clicked the X on the righthand side of the devices name. That made the device disappear. I restarted the iMac and checked the bluetooth devices list again. It was gone from the list of bluetooth items.

Maybe you could try that.

Great, I should have suggested it first!