One of our ground handlers (who is a PPL) told me of friend of his landed at YSSY in a 172 a couple of months ago and taxied around and left again.
Maybe your Duchess didn't have the right PPR (although as you say, with it so quiet there's a questions as to why he'd need it)....
It's scary out there. Early evening yesterday, I got direct to SOSIJ from 140nm out, "any speed you like".