Originally Posted by Garcia Bronco
He wasn't a suspect in any particular investigation that I am aware of.
He was however walking through a residential area with a growing crime rate. This is going to cause increased scrutiny of unknown people cutting through the area.
Furthermore, Zimmerman knew (or at least thought) that Martin didn't belong or live there....since he kept tabs on the neighborhood.
Nothing at all wrong with asking 'what are doing here'?