A fibreglass aircraft with transponder turned off must be close to invisible. If its parked on private property with a non obvious strip it should be very difficult to detect.
You need to visit ASA sunny, they can pick up a flock of birds heading your way so a Jabiru with no transponder is gonna stand out like dogs balls.
What sane punter / member of the public is going to come to some unmarked strip and hop in this fibreglass aircraft for a joy ride? You're dreaming.
At best some non AOC holder is going to start illegally carrying some freight or an AOC holder doing IFR flights under VFR.
It's probably the legit operators taking short cuts that you ought to be worried about.