A question mark
A question mark was a written symbol that indicated that a question was being asked.
In 2375, at the end of a The Adventures of Captain Proton holoprogram session, a question mark appeared on a screen. (VOY: "Bride of Chaotica!")
In 2381, the Truther recruitment booth on Tulgana IV had a question mark as part of its display. (LD: "Reflections")