When the chief stops spinning it means that a rare collectable will appear somewhere on the map. S/he doesn't point to its location. It will be rare, but may well be a duplicate, one you've already found. The normal appearance of collectables isn't affected, so if you've found non-rare items they weren't the ones pointed out. It can be difficult sometimes to scan the whole map before the rare collectable disappears, especially as some tend to blend in with the background, which would explain why you probably haven't always found one.