Armor Proficiency Check on Armor purchase--where?

[Paragon]

A while back I went in and fixed the fact some of the latter-day armor proficiency feats were not enforcing their pre-requisites, but I just noticed that the actual armors (Studded Leather for example) were not actually requiring their proficiencies, so I just pulled up the an armor to see how they were being enforced--and there seems to be nothing there that's doing it, but Plate Armor (for example) is clearly enforcing the need for its proficiency. Anyone have any idea where this is being done? It doesn't seem to be under the armor itself, since there's no pick-reqs or expressions under those, so I've got to assume its somewhere else.