Conan Doyle Great Boer War "Returned Empties"

Supplies for front line troops were shipped in crates and barrels. In order to save time, money, and material the empty crates and barrels were returned to various freight depots to be refilled and then delivered to encampments in need of supplies.

The phrase was also part of common vernacular as most glass bottles had a deposit and so had some intrinsic value, but no way near as much as the filled ones.

The phrase implies a waste of manpower guarding what was basically recycling and was most likely to be returned back to Britain to be reused.