Here's a video with evidence from NASA's files. The narrator is a bit on the cheesy side, but the info is there.
There's another video, too with audio of the shuttle astronauts notifying Houston of a "bogey". It becomes obvious in the conversation that what they're talking about wasn't supposed to be for public broadcast and they get hush about it and then switch channels. I can't find the original video, but
here's another that does play that portion of the conversation along with some other testimony.