Blocked account email "Access expired" scenario

Hi, we are implementing a handler for when users follow a link in the blocked account email template, resulting from the Brute Force attack protection, as documented here: Email Template Descriptions

One of the possible scenarios listed on that page is “Access expired”. Could you clarify in what conditions that would occur? Do the unblock links have an expiry? If so, how does a user in this state unblock their account once their link has expired? Or does it mean that the user has since been unblocked by some other mechanism and therefore the link is no longer relevant?

Many thanks for your help