I did notice that some knife collectors have some kind of an insane eye for detail on knives.like a sherlock holmes looking over the enitre blade looking for 'a spot'that may be the weak chain ,where the whole knife might start to crumble down.
the normal thing becomes an obsessive abnormality.(sorry for my english,it's not my main language)
I wouldn't worry about that spot.
it is molded.it is normal to have a spot like that on rubber.
I bet ,when you just use the knife-handle like one does when using a knife, the spot still looks the same after 15 years.