In this case, it was likely an electric socket.
Over the past several months we've seen a few Tesla Model S fires, most of which were caused by the car colliding with something on the road. Fortunately no one was hurt in any incident, but the NHTSA has still opened an investigation to figure out what exactly is going on. In the meantime, some conclusions can be drawn regarding the Model S fire that occurred on November 15th in Irvine, California. A garage where a Model S was parked caught fire, which led to speculation the car itself was to blame.
Tesla investigators, however, now believe that the car battery didn't overheat and start the fire. They're not sure whether the fire started in the wall socket or if it was caused by the charger, but the battery is definitely not to blame. The car didn't have any fire damage, and "the cable was fine on the vehicle side; the damage was on the wall side."