Developed a rattle a week ago and found this when I changed it.
Its the aircon tensioner

It was a cheap part, so not surprising or the cage has disintegrated
New one should be good for a good while.

I doubt you've lost any balls, most likely the cage broke up and fell out so the balls all ended up on one side. Had that happen to an alternator bearing many years ago (on an old Talbot) and ended up chopping bits of nylon off a tie wrap and jamming them in between the balls to keep them spaced out around the bearing. Lasted about 400 miles which would have been good had I not been 700 miles from home at the time......