Watchman for Mac is a utility that can scan a single page or a whole site on schedule, it'll archive the page(s) and alert the user to any changes in the code or visible text.
We've just made a version 2 beta available. It features a 'non-UI' or headless mode, which means that it can remain in the background and not interrupt the user when a scheduled scan starts. Its windows can still be accessed from a status bar menu.
Version 1 is still available. It's free and will remain free. The new beta is also available and free at present, but version 2 will eventually require a licence.