It's not that it doesn't let the Can bus sleep it just attempts to draw current to charge itself. The siren is a control unit in its own right and communicates via Lin bus single wire. If the master unit cannot see it then it continually sends a signal looking for it and also let's it draw current from the battery to charge itself, until the voltage stored reaches a set level. With a faulty cell this cannot be achieved, hence the drain - continuous draw.