Unfortunately, I live in the city. One thing cats learn right away is that it's easy to slip through certain types of fences, and humans / dogs / cats / other scary things aren't able to get through those fences. In the city, fences are EVERYWHERE. And those fences often have locked gates, and neighbors who are unhappy when you break into their yard.
Anway, various pet locators:
Or, do it the other way.... find some stimulation that the cat recognizes, and will be attracted to. (yes, I swear I can train my cats... japanese fish shavings are remarkably powerful attractors)