Was a flight plan filed and who with? why did atc not pipe up and say something? (not blaming them at all) but even though they would have been out of their air space, you would have thought somone would have noticed? IFR in single piston? is that allowed on charter?? not the last time I checked. Even in positioning flights when I used to fly a 206, atc would not allow it at IFR levels?
