Sorry, but I'm still not convinced about the vandal theory. Like GzRider said, you've got to know exactly which plug it is, then contort yourself into an uncomfortable position under the bike, then undo a badly designed plug with the correct wrench. I've often convinced myself that there's nothing I've forgotten (like torquing it up tight) then having doubts, some time later, & finding out that those doubts were well founded. It's just one of us fallible human being things.
I reckon a vandal is more likely to do something easy, like one did to mine. Chop the starter motor cable, which is quick, easy, and unobtrusive to do.