Trivia: Will Smith has his own signature line, which is called 'Oh Hell no.' He says it at least once in almost all his movies, including this one.
Trivia: In the beginning, the song "Move B*tch" is accompanying Hancock's first action scene. Will Smith sings this song in "Bad Boys II" when threatening the boy who comes to pick up his partner's daughter for a date.
Answer: I saw it, too, and wondered. He's a street performer in L.A. that just happened to be around during filming. See: http://io9.com/5023487/hancock-giant-robot-mystery-++-solved.