Actually since he didn't see the REASON why you parked in front of his house, and he DID know you'd been detailing it, he probably came home and saw it parked in front of HIS house knowing full well you could have easily parked at your own. So in his mind he probably thought you were making some kind of "in your face" statement at him and that's why he turned on his sprinklers.
You might try just going over there and talking to him and explained why you parked there instead of descending into an escalation war that tend to get out of control and result in much more expensive issues than just some water spots.
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