Some of the posts here mention bells or horns that are used for other purposes, such as calling the dog. The important thing about an emergency signal is that it is only used for emergencies. I am liking the idea of an air horn. Even if the neighbor is used to hearing the racket of the shop, they will probably notice a 5 minute blast from a horn and would hopefully investigate. If I ever move the shop out of the attached garage, I will install an air tank and pull cord horn, or at least get one of the little canned horns. A 12v car horn on a battery would work. I guess you could even hook-up a car alarm and keep the remote handy.
Jerry