One of the nuts that connects wires and the board had come loose and an area around it of about 4 square centimeters had turned black.
This has to be a Maintenance Control or a Quality Assurance issue. The work on the electrical panel was either not documented properly or not inspected properly.
Loose high current electrical connections are a well understood problem in commercial electrical maintenance. The loose connection begins to heat, the metal begins to oxidize, the resistance of the connection increases, the heating increases, and pretty soon you have metal at the scene of action being vaporized, but very little current flowing through the connection.
I wonder if they are doing infrared surveys of the electrical gear to look for developing problems?