From the experts themselves, the
Boeing Flight Crew Training Manuals, no less:
"To reduce the possibility of flap damage following an approach in icing conditions or landing on a runway covered with snow or slush, do not retract the flaps until the flap area has been checked free of ice debris by maintenance."
So it's a normal practice all right - for icing contamination , not hijacking! I really don't know where the latter idea came from - military? Certainly not written in my books.
Do you really think the tower/ground would even notice and potentially call the Swat team? I really don't think so....
Oh, and notice it is not just when you land on contaminated runways, but also when you
conducted an approach in icing conditions - something we tend to overlook, I suspect.