On logout as admin, PHP code from node pages is executed.
If the page you are referring is a node page, then the node could be using the PHP input format, and in that case the node body would be passed to eval() through drupal_eval().
If the page is a view page, the Views module allows the administrator users to enter PHP code that is executed, through drupal_eval(), in specific circumstances.



In both the cases, a block visible in the page could be using PHP code to set it block visibility.



There could be other modules that call drupal_eval(). Those modules pass to drupal_eval() code that users with the right permission enter in the module's settings; if you have one of those modules, you should remember ending PHP code in a text area.