The purpose of overpack labels on ADR

Evening all.

I’m relatively new to ADR work and im after a definitive answer as to when an overpack label is to be used on a ADR pallet.

I’ve raised the question after having a bit of a disagreement with my employer about a pallet that I collected. The pallet in question was a mix of LQ and a small amount of Cat 3. The Cat 3 was a small drum which had been placed in amongst the LQ boxes and could not been seen. The pallet had the UN number on the outside of the pallet and the Cat 3 diamond on clear shrink wrap. I argued that the pallet should have had an overpack label on it as the drum couldn’t been seen, while my employer said it shouldn’t.

Any help on clarification on the use of overpack labels would be great.

Cheers