All physical locations (buidlings, mountains, exits), except events (fiestas), take estar. The whole 'permanent vs. temporary' thing does not apply.
Rspreng, I have been reading your comments since I started DL several years ago now, but this one gets a lingot for its conciseness, clarity and correctness. Happy New Year! ¡Feliz Año Nuevo 2017!
Something that really helped me was to stop thinking permanent vs temporary and focus more on state vs essence. The location of an exit sign is its current state, not part of its essence. The location (state) may change and doesn't really make up what the exit sign is (essence). Its it red and white, square, plastic, etc would describe what it is (essence).
I think location should just be thought of as a separate thing which requires estar for things and ser for events. To say that being in Spain is not part of the essence of Madrid ... that it is a Spanish city, seems absurd to me. Just accept that estar is used for places of all sorts and move on, don't try to rationalize it any further.
An "exit" is a discrete object that is designed and/or intended to be used as a means to leave a certain space. A "way out" doesn't have these requirements. If the exit of a room is blocked, there is no way out, even though the exit is physically still there.