He was "off (voluntary) duty" when he saw Trayvon. Zimmerman was also on his way either home or to an errand.
I don't have any issues with him having suspicion. In the tape of him speaking with police dispatch, there is a period of what seems 15-30 seconds where Trayvon is aware he is being watched, and his reaction (according to what Zimmerman says) is to stare at Zimmerman. All this while Zimmerman is sitting in his vehicle. Then Trayvon takes off at a run.
If that sort of thing was going on in my quiet neighborhood, I'd be suspicious too. Burglaries aren't quite as frequent here, but they do occur.