Honestly I get it, there was a guy who owned the most obnoxius chevy truck in my area for at least 3 months, no exhaust anywhere to be found and it was LOUD. He wouldn't just drive slowly down the road either, it was redlined everywhere he went. Didn't matter time of day (or even if there kids playing near the road on their lawn). His driver door one night sprung a bullet hole and that was it, peace and quiet from then on.
Guys like that always get justice in interesting ways.
I'd never touch another persons car/truck, but at least in OP's case the oil slick was left there purposely as a clue. No damage done to the truck, just a subltle message: "next time it will hurt".