If you visit a website (a domain) for the first time, and that page doesn't load the tracking script, you'll have an empty entry for that domain (listed as "no scripts") on the options page.
However, when you visit a page on the same domain that has the tracking script, the list of scripts doesn't seem to get updated.
I manually deleted the entry for that domain from the extension's local storage, and it updated correctly on the next page load.
I think that in background.js (in the extension), the checkForScripts() should be called more often (not sure how often though).
Thanks to a very helpful customer.