'tis truly amazing how many washers, nuts, bits of lock wire, cotter pins and, previously blocked, drain holes you find in the bottom of a fuselage when you clean 38 odd years worth of grime out of them.
Since I have personally run into a screw falling from an aircraft above, I'd believe it.
VH-MEP, Jun 11 1986, Over Ardrossan.
Result...hole in windshield and buggered prop blade.
Aircraft that dropped the screw was a Tobago from AFTS.