Popularized by Antoine Dodson, who used the phrase to increase awareness of the fact that there was a rapist in his neighborhood.
Since that time, use of this phrase has been increasing, and it can be used at any time, when an obvious fact needs to be pointed out.
Other examples in common speech:
Janine: What happened to my money I left on the bar?
Bruce: Well, Obviously, we have a thief in Snuffy's Bar.
Janine: Oh wait, I spent it all on wine.
Bruce: Well, obviously, someone has drank too much.