Previous house had intermittent fault - drove us mad... tripped usually early hours... turned out the dishwasher leak 2 months earlier had finally seeped through tiles and screed enough to get moisture inside a hidden junction box laid in the insulation below the gf slab. The condensation level increasing during the night was just enough. It looked like the builder had smacked through the ring main conduit and cable, bodged a repair and concreted it in! Having traced the dodgy leg we bypassed it and only found the “repair” nice and moist months later when we took part of the slab out for an extension!