That flat piece of rubber is the anti backflow flap that most likely came either just before or just after the drain pump, and the little "handle" is its mounting hinge. You will have to look in the end of the hose that you disconnected or the piece of grey plastic pipe to see if there's a corresponding molded in spot for that hinge. If you don't find a mounting spot there, then it must be located in where the water enters the drain pump from the tub.