Geoff wrote:Mind they chew up paper and plastic as nesting material so they might have found food by chance.
I can vouch for that: when my OH was dry stone walling, he often found mouse nests comprising chewed up crisp packets in the walls, presumably having been dropped by hikers or, as is their habit, the hikers having stuffed them into wall. The mice then made a meal of the bits of crisps (and the salt?) and used the shredded packets as nesting material!