If you're doing something annoying or illegal they could always follow it and see where it lands. More likely they would ask when they drive by and see someone

If you're doing something annoying or illegal they could always follow it and see where it lands. More likely they would ask when they drive by and see someone preparing to launch one. There are lots of hunting, firearms, and vehicular regulations that get enforced by cops (and other peace officers) who see things while on patrol.