My iphone X with latest iOS says 'Location Shared by another device' when I open Find my Friends, but I have no other device. I also have 2FA enabled so I'm not too worried about someone logging in as me, just curious how to go about solving it.
Once upon time, not long ago....
You shared your location with Family or with a Friend.
How to remove old friends from sharing location ?
Launch the Settings app on your iPhone or iPad.
Tap on Privacy.
Tap on Location Services.
Tap on Share My Location.
Tap the friend you would like to remove
Tap Stop Sharing Location