Is it possible that Darth Vader somehow has access to C-3PO’s visual and audio data?
Whether you come to the conclusion that he is an Imperial spy or not, is entirely up to you.
He does serve the Rebellion well, but at times his motives and behavior are questionable, which has lead to this theory being investigated.
That would essentially mean that C-3PO is constantly feeding a stream of information back to Darth Vader, helping him to evade capture and know the Empire’s next move.