Anti-loitering device

Walking by a local high school, there's a sign posted meant to discourage people with no business there from hanging around. The reader is warned that certain "anti-loitering devices" will be brought into play to move out loiterers.

I wonder what sort of devices these things might be? I have a mental image of a husky little machine on tank-like treads, sporting a whirling radar dish on top. Maybe it'd have mechanical grabbers on the front, or flame throwers.

That'd discourage this kid!