I agree there are reasons a guest instructor might be there to check things out. But correcting large numbers of things in a single day isn't likely to have any positive effect when the regular instructor will be back teaching that class next time. Those corrections - if the point was to check out the instructor - should be given to the instructor or CI, or to someone in the organization.
As for him being newly graded, the oh-so-stiff belt that has to be bent into a knot, rather than tied, usually gives that away.
He may not have just been at that session, the OP only saw him at his son's. The guest instructor may be the 'secret shopper' of that organisation.
Our belts were never stiff as we softened them before tying, took a while rolling and unrolling them as well as 'scrunching' them but nobody wants a stiff belt! It's not cool!