A sign, or indication, was an thing, event, or individual whose presence or occurrence indicated the probable presence or occurrence of something else.
In 2254, the USS Enterprise received a message saying that the SS Columbia was making a forced landing. As there was no indication to the positive that there were survivors, Captain Christopher Pike choose to keep the Enterprise on its current course to the Vega colony. Later, the starship received a follow-up message saying there were eleven survivors. With this latest development, Pike ordered a course change to the Talos star group. (TOS: "The Cage")
In 2265, during a briefing in which the mutating Gary Mitchell's abilities were the subject of discussion, Montgomery Scott and Spock described an event in which Mitchell was playing with the controls of the bridge by shifting levers, pushing buttons, and changing instrument readings. With each change in the bridge controls, Spock, who was monitoring Mitchell, saw him smiling, as if, in the opinion of the science officer, the ship and crew were a toy for his amusement. James T. Kirk asked Elizabeth Dehner to confirm the account of Scotty and Spock and if Mitchell had "abilities of such magnitude". Dehner had seen indications of these increased abilities. (TOS: "Where No Man Has Gone Before")