taken home ordinance from an exercise
One imagines a small amount of personal ammunition, not 340 rounds, and this happens a lot. Rather than taking it home, and risk getting caught, it was common to slip the spares into the gullies on camp or in to the skip. Contractors continued to complain of live rounds turning up even AFTER amnesty bins were provided. It was often easier to discard the stuff illicitly probably as it was found some time after it should have been put in the amnesty bin.
Or the RAuxRegt FS that simply stored unused boxes in the unit lockup rather than dragging in the regular duty armourer and battling with the paperwork late on a Sunday night. It also meant their consumption was recorded as at the permitted rate and they would have spare ammunition for the future.
Squirrelling 340 rounds away smacks of convenience rather than need.