Have your 1DX2. Just pull out any card(s) you have in camera. Just leave the camera in Auto-Off mode (do NOT turn it off from the on/off switch).
Check again in few (two?) days.
Battery = alive.
Battery = dead
(...or heavily drained out)
IIRC also 5D3 has/had this problem...
I think that a firmware update can fix/enhance this thing.
TIA for the fix 🙂