Not EOD, nor did I even spend the night a Holiday Express, but my GUESS would be to anchor what ever it was to be a part of to something, out of sight.
Since most metal in an aircraft is aluminium, or other light wieght non-magnetic alloys, not steel, that might provide a clue as to where this was intended to be placed...to start looking for things in the future.
a small simple magnet can be used to activate a reed switch. They can be of the normally open or normally closed type...ie activate when the magnet is brought near or activate when the magnet is removed. Reed switches...a common very small silent componet even available in a plastic bubble pack at radio shack. Been used to activate bombs etc for years.