Yes, it’s definitely on the inside
We've now become convinced it can get into the back room, when I discovered a lot of shredded plastic bag. There was a plastic sort of tier of shelves standing on the floor, and the middle one had Jess’ hiking/survival kit in it. Amongst this lot was a supermarket carrier bag with some of those little tubes of coffee/chocolate that you add hot water to when you want a hot drink. They’re very light and convenient when you’re on expeditions. The rat could evidently smell them (how strong a sense of smell is that?) but had approached the shelving from the back which is sort of slatted. It managed to pull part of the carrier bag through and then kept pulling and gnawing at it in an attempt to get to the food. It made a lot of ‘confetti’ on the floor and pulled all of the bag that it could through but of course the actual tubes were too big to fit between the slats so it eventually gave up and went away hungry. Well not quite that hungry because as already noted elsewhere it is partial to a bit of plastic.