What do browsers do with expired cookies? I know that the app won't read them in, and I'm assuming the browser wouldn't send them, but are they purged from the file system with any regularity?
If a cookie has expired, the browser does not send that particular cookie to the server with the page request; instead, the expired cookie is deleted.
This depends upon the specific browser and also the user's personal settings. Welcome to the world of web development, where everybody is a special case. This is one of the reasons you need to test your site extensively, with multiple browsers, from different locations.
Browser checks for expired cookies when a page is requested from that domain. If a cookie has expired, the browser does not send that particular cookie to the server with the page request, and deletes it.
If a page has not been requested from that domain, the cookie will reside on your browser till it is overwritten by another cookie (happens when memory allocated for storing cookies gets used up).