Also never heard of it happening here in Oz, at least not on the grapevine. The nearest to this I have heard of is when instructors have have had to do the sudden tug to pull the cyclic out of a panic stunned student with a death grip on the cyclic. Kind of the reverse to the one being discussed.
Most people remove the Pax cyclic and collective etc when not using it. Have heard a few times where the collective has been interfered with, usually by the seat belt being looped over it by Pax but you find out about that on lift off, it isn't a thing that suddenly appears in flight.