I would imagine the Echidna would have left as many marks as they could around the civilisations to alert everyone of the dragon born of water. Including what he looked like, so anyone who saw would not make the mistake of ignoring him for not being a dragon. Robotnik does indicate he has some research regarding Chaos, since he knew to destroy the Master Emerald to free him and refers to him as the God of Destruction. Considering the next game has him attack G.U.N. for the Ultimate Lifeform, he might have broken into their systems at some point and go the ARK Project Research Database or something to that effect.