Perhaps there's a problem nearby with porcupines or other critters? I know that in many areas, people parking their cars are cautioned to use a variety of tactics to keep various critters away from their car. Most people use moth balls, but it seems I've heard that Irish Spring soap keeps something or other away.
(Porcupines will chew up your brake lines & other wires. Apparently, they're yummy to porcupines.)
edit: I did some searching. Seems that Irish Spring soap, used in conjunction with a couple other things, keeps deer out of your garden. So, that wouldn't make sense for the engine compartment. Since he left it open, mice, maybe? Hoping to get rid of them?