The plugin will contact the PLN staging server ever 24 hours to check for new terms, and before sending a deposit. If the terms have changed then the old agreement is cleared, the deposit is stopped (if there is one), and a notification is queued for the journal admin. The notification will be displayed when the admin views a journal management page (and possibly any page in the journal).
Thanks. I had missed the call to PLNPlugin::getServiceDocument() in Depositor::executeActions().