Although you can say kiu and tiu for both things and people, they are specific things not unknowns or generalities so in practice i suspect if you see a kiu sitting out on its own like that the probably intention is to ask who? Ie a person. Other opinions are probably available and i wait to see what others think. ;-)
well, it might be "which other things are here", if we were already talking about things; but if you walked into a room and said "Kiu alia estas ĉi tie?", you meant "who else is here"
I think of kiu as adjective-ish, and when it doesn't modify a noun, it defaults to meaning a person ("who"), or picking out of a set that we've already established ("which")