I've found this a few times when I've pulled breakers out of the ITE style panels... It almost seems like either the contacts in the breaker didnt completely make & they heated up to London Broil temperatures, or corrosion occured between the breaker & the contact stab in the panel & thus lead to a heated connection... I've yet to come across this leading to an actual "structure involved" fire call... But none the less, evidence of that kind of heat having been present does make me think to "double check" all those connections & lugs...

Randy