It's almost impossible for Youtube or others to find if this has been done without brute-forcing things.
Suppose you match the least-significant bit in (say) the green byte of every (say) 31st pixel in the video file to the next bit in your data file. Then you merely have to extract that bit and append it to make a new copy of your data file. Yes, you would need a very large video to store a quite small data file, but it could be done.
Youtube's problem: Which particular bit of the 24 in a color-pixel did you use? Was that pixel every 21st, or every 31st or every 101st in the video file? And which particular video files out of the millions added every day were ones with an embedded data file? The permutations rapidly make it an impossible task.
