300tdi rear brake pad missing..

had a grinding noise coming from calipers thought the pad was worn,took wheel off to have a look and found pad had come out at rear of caliper..is this comon if so how can i stop it happining again as pins and clips were still there just no pad..

The friction material had come away or worn away from the metal pad and so the metal had just droped out in to the gap between the piston end and the disc. Cheepie pads have been known to split this way. So fit decent pads and always replace the spring tensioners the rear. U can't loose the pad at the front as there are pins with anti-rattle springs that go through holes at the top of the pad.