He doesn’t know what Martin was up to. All he knows is that someone is standing on private property looking at houses. The same property that was believed to have been broken into and near property that police caught people with stolen items on them. Again, that theft was stopped because someone else noticed people standing around in a suspicious manner. Zimmerman is actively trying to prevent more crimes in the are and when he sees something suspicious he calls the police. It’s no different than what is expected of you if you see someone doing something suspicious at an airport or a marathon. As I’ve mentioned before, I stopped a theft in progress by observing a kid standing on the sidewalk. It was the observation of his behavior that tipped me off. And he was about as close to “Timmy on the way to Disney” as you could get.
There’s nothing wrong with that if that’s what happened but that is not what Zimmerman reported and this goes back to what I addressed. Martin took an inordinate amount of time returning home. That doesn’t mean he was looking for houses to break into but it does add to the background of him loitering about and that is what Zimmerman reports.