LONDON, June 12, 2002 — If you heard anything, you would think it was a mosquito hovering, hunting for fresh prey. But in the dark night skies over the Balkan mountains, that distant, faint buzzing may mark a hunter of a different sort. Shrouded from view, loitering up to 16,000 feet in the air is a small army of robot spy planes used by Allied forces to watch for trouble. Every day, the spy planes are aloft to monitor the high mountain passes and deep valleys for illicit traffic, across routes in use for centuries to smuggle arms, drugs, even women destined for the sex business. In Afghanistan, some of the spy-in-the-sky observers can even be armed to fire missiles by remote control.