In addition to using our API, the browser extension looks for a PDF link directly in the page content and, if one is found, displays it as a Bronze OA icon. If you're logged in to a publisher site or on a university network with access to a paywalled article, this can be confusing as it doesn't reflect the article's true status.
This functionality was added because the plugin is focused tightly on helping users legally obtain access to fulltext, one way or another, rather than educating them about access. We don't want to show a "no access" icon and have you miss something you could have read when the link was right in front of you. If the details of the article's status are important to you, we suggest using the API directly.
Of course, it could also be a bug. If the link we're providing doesn't work, please let us know about it at https://support.unpaywall.org or email@example.com.