So I changed my differential fluid the other day, and found about 15 to 20 of these metal chunks . It looks like the tips of the diff teeth to me.
Background info: 02 wrx 84k miles, completely stock, tires are cupped and one has excessive wear on the inner tread. I feel a very mild skipping feeling sometimes but its only when going straight, or at least I don't notice it while turning. I don't hear any weird noises coming from the diff either.