actually it is the business of air traffic control to make sure that a plane of any kind on an IFR clearance is complying with the clearance.
He might have closed his IFR flight plan before, he might have declared "airport in sight" in which case the ATC is done with him. The primary responsibility of ATC is separating the traffic from each other (and separating traffic from terrain when on IFR flight plan) everything else is done on time-available basis.