Then that raises the question as to why Zimmerman was in the location he was, and how he end up parked up on Twin Trees Lane long enough to become aware and then suspicious of Martin, plus the length of the phone call to the police:
[qimg]http://www.nickcooper.org.uk/personal/images/jref/tm_aerial.jpg[/qimg]
The red line shows Martin's probable route back to where he was staying (blue marker bottom-right); the small purple rectangle is the approximate position of Zimmerman's vehicle (although it may have been slightly further to the right). The blue marker slightly obscured by the red route line is where the shooting took place, and Zimmerman's house is the blue marker bottom-left.
Let's assume that Zimerman was "driving back from the store." If he came in the "back entrance" (bottom-right), the obvious route would be to take a left onto Retreat View Circle, and then follow it round to his home. Alternatively, he may have driven straight across to the lower part of Twin Trees Lane, then along Long Oak Way, and a left onto Retreat View Circle. Either way, he wouldn't have been anyway near where his vehicle ended up.
On the other hand, if he came in the "main entrance" (top), you'd assume the most obvious route would be to take a right onto Retreat View Circle, and follow it round. Alternatively, he could have driven straigth across, onto Twin Trees Lane, then Long Oak Way, and then the last bit of Retreat View Circle. The latter is a slightly longer route, and is the only one that would put Zimmerman where he ended up, but it still raises questions.
Most importantly, Zimmerman's vehicle would have been facing away from the direction Martin was approaching from, on the opposite side of the road. This would mean that Zimmerman would either have to be turning round to watch Martin, or he was using his mirrors to observe him - either being details missing from what we know Zimmerman has said.
One possible explanation is that Zimmerman may have first spotted Martin walking onto Twin Trees Lane as he (Zimmerman) drove in the main entrance. That may have caused him to carry on to Twin Trees Lane himself (rather than turning onto Retreat View Circle), and then park up there ahead of Martin.
It really doesn't seem credible that Zimmerman just happened to have stopped where he did on Twin Trees Lane (for no explained reason), just as Martin appeared. Either he was parked up there specifically looking out for the burglars he was so concerned about, or he spotted Martin earlier and changed his route and stopped to observe him further. Both put a very different slant on Zimmerman's actions than him innocently "driving back from the store."