Then something somewhere in your stack/your machine is awake on 8080 and serving content. But only to GET URL’s
(A released extension won’t be able to POST as navigation is blocked/not permitted, but thats outside the scope of this thread)
Follow the iframe tree and see whats being loaded from where
So if you right click the extension and hit inspect.
You can follow the elements up and check the URL that is in use on the iframe
Then visit that URL directly and see whats returned.
If it’s loading when your server is off, then you have another server serving content responding to GET requests.