Is the discoloration due to overheating?
That’s a good potential reason.
Any other clues?
No there were no other clues of over heating at all. Thats why i posted it here. I was confused. Any suggestions for the write up?
Report what you see.
Evidence of apparent overheating at main breaker bus. Recommend qualified profession determine cause and cure as needed.
simple enough 
Aluminum?
Has the entrance been upgraded?
SUSPECT. Over heating breaker on the ( left or right ) bus bar.
Number of the break if you can in a illustration or in the photo of the open panel…
Recommend a licensed registered RBQ ( my province) certified electrician determine the problem.
Just my 2 cents if it helps.
Sorry Larry. Just saw your post.