Undiscovered Drops from creatures - how does it work?

How does this work? How can we "discover" these undiscovered drops from creatures?

For example i fought Old Guard and dropped Clay. Very exciting since we need a lot of it.

But when I go look at Old Guard in db, it doesnt show he can drop Clay.

I thought when i drop an item from him it would then be added to the list because i "discovered" he drops it. Does it not work like that?