If its just one in a an otherwise rabbit free zone, shift the straw and dig back along the hole until you reach the end.. If no rabbit, just leave it open for a while and then if no activity, fill it in. If you are within an area containing lots of rabbit workings,it is probably a bolt hole, ie an emergency exit to the main colony. Are you on a managed site, where pest control is permitted or are you out in the rural heartlands where ferreting etc is the norm?