User blog comment:Data-sora/Goat/@comment-3411077-20120708221350

Actually, there's a whole bunch of hidden things and easter eggs in Gravity Falls. (I also must remind you that Adventure Time is not the only show to have a hidden character or object throughout episodes.)

1) There's the mystery man that people have already mentioned.

2) There's a quick glimpse of bigfoot at about 15 seconds through the theme song.

3) "I'm still here" could be heard whispered very faintly in the background at the end of the theme song.

4) The cute biker guy is seen sometimes in the background.

5) Etc.