I think it is an EA oversight at the minute as pickpocket in general is buggy and always has been since day one. The NPC "should" require to pass a check to notice something is missing and only if said NPC is in a position to notice said theft. Stealing a bunch of gold from a pocket should get noticed very quickly while someone missing a ring off a display case should go unnoticed unless the NPC has reason to look. I can imagine this level of coding is convoluted but it could be more easily coded with regards item weight/value and story relevance. If you have a "dagger of the crusty muff" worth 2000 gp next to a "used crusty jizz sock +2" worth -1 gp an NPC should pay more attention to the valued stock etc.

In D&D it would be the GM rolling the dice so it's a hard one to quantify at a game level.