OK, let's not use terminology that doesn't apply directly to Joomla. Joomla has no areas called frontoffice, or backoffice, or internal, or intranet.
Joomla has 2 distinct areas, /administrator and the site itself. Either you're in /administrator, or you're not. In /administrator, my plugin does not operate - although you can lose your cookie if you browse the site and /administrator at the same time.