Rat shot instead of solid lead from the gun next time will help. Most of the time they will not leave until their food source is gone. Seems like the shed with underpinning may have been a nesting area for mice that they are feeding on. Also the snakes like areas they can get into that are dark-underpinned area again fits this-and feel protected. Take the underpinning off and hope for the best. Just make sure that you have a gun handy to take care of the ones you see when you take it off.