Looks like it doesn't detect that page because it got submitted without the trailing slash, and the page forcefully redirects to the slashed URL. However, that's easy to correct for, and I've updated the extension to check for both versions of such a URL. Thanks for trying it out and finding this bug! If you're still using the extension, it should auto-update soon.

Re: the little side tab could tell me implicitly that someone else submitted a story I stumble upon

That's exactly what the extension should do; if any webpage you are looking at has ever been submitted to HN, the orange tab will pop out.