No rule has ever been stated by Dr. Okrand and he is not completely consistent in his own usage. However, a careful examination of the canon shows that Dr. Okrand does have a general tendency towards when and how he uses -taH on a pronoun as verb to indicate the location of something. If the subject (the thing being located) is a permanent structure (like a building, a restaurant, a tree, a river, etc.), then he tends to leave -taH off. If the subject is a mobile object (like a person, an animal, a weapon, a food item, etc.) then he tends to add -taH. I'm not sure how to treat objects which can be moved, but generally stay in the same place for super long periods of time (like tables, art work, temporary sign posts, etc.) - perhaps whether you choose to use -taH or not show your mindset as to the permanentness of their placement.
I actually like the suggestion you give in another sentence topic: that -taH indicates a thing is in a somewhat temporary location, and leaving off -taH means a thing is in its proper place. It would explain some strange examples, like nuqDaq 'oH ngop''e' Where are the plates? in which the plates are not necessarily always in a fixed location but the speaker is asking about the normal home of the plates rather than the current location of the plates. It would be equivalent to nuqDaq ngop Dapol Where do you keep the plates?
But in the end, this is just a guess, and the examples in Duolingo regarding this have been built from someone's guesses. Don't take them too strongly as the way things must be.