Are you sure ALL loads have been disconnected?
Not just turned off but unplugged.
Yes, it's possible the breaker is faulty but it's also possible there's a wiring fault.
Many years ago I had a washing machine plugged into an unswitched outlet that was switched by a wall mounted switch above a worktop. One morning I discovered the walk switch was warm to the touch. Investigation showed the Neutral wire inside the switch box had been pressing against the head of the screw holding the box in the wall. Over time the insulation had gradually crept under the pressure and the neutral conductor had contacted the screw and therefore the (earthed) wall box. An earth leakage breaker would have tripped on that.