Whenever the logout() function of Mage_Customer_Model_Session called then

Magento is assign the customeredcustomer as guest using $this->setCustomerGroupId(Mage_Customer_Model_Group::NOT_LOGGED_IN_ID);

ThatFor this reason at magento,you can see the customer as guest use***the customer as guest ***. also as per as system, after fire of customer_logout event magento is set customer as guest.So if you tried to make change then that may be not reflect

protected function _logout()
{
    $this->setId(null);
    $this->setCustomerGroupId(Mage_Customer_Model_Group::NOT_LOGGED_IN_ID);
    $this->getCookie()->delete($this->getSessionName());
    return $this;
}

If you want to remove those assign then just rewrite Mage_Customer_Model_Session

and remove the code:

$this->setCustomerGroupId(Mage_Customer_Model_Group::NOT_LOGGED_IN_ID);

rewrite class:

<?php
class  [ModuleNameSpace]_[ModuleName]_Model_Session extends Mage_Customer_Model_Session{

 protected function _logout()
    {
        $this->setId(null);
        $this->getCookie()->delete($this->getSessionName());
        return $this;
    }

}

This may be help you.