Blades can indeed flap at low RPM, every rotorcraft pilot should know that, but I don't see how that would cause the helicopter to lift up unless the wind is REALLY strong. With the collective down, the blade pitch is 0 and the blades are generating no lift. Centripetal force is straightening the blades, not aerodynamic lift. An idling helicopter generates basically no downward air. People can enter/exit an idling helicopter easily without feeling like they're in a hurricane because it's not blowing air downwards, hence not generating lift.
Likewise a parked airplane generates no lift. However there have been occasions where extremely strong winds have actually made upwind-facing airplanes go airborne if they're not tied down, so the same could happen to an idling helicopter, but that would have to be a really strong wind of the sort you would never fly in anyway.
That said, personally I wouldn't ever be comfortable leaving an unattended helicopter running, but then again I'm not even comfortable leaving a car running unattended.