Does it look like someone was in there before you received the units?
I have seen sloppy work where the pad was not lined up properly and the screw cut a new (now larger) hole in the silipad.
It does not surprise me that there were carbon tracks if the hole was too big or there was a burr.
Can you say "arc welder"/
And if there was a burr on the heatsink, old welder vaporized that.