Got any pics of your Leaf battery setup? If they are like my Gen 4 battery modules they are lithium bags in a basic vented shell( thin metal shell over the bags with some plastic for the top connectors and at the bottom). All kinds of road debris can get up into the rear seat battery compartment unless the floor has been blocked off.
Mine look like this: Nissan Leaf Gen 4 battery conversion - #65 by dougl