4
a module's xoops_version.php is read when the XoopsModule::getInfo() method is invoked.
This method is invoked on module installation/update/uninstall and by the main menu block - for active modules with a weight above zero and a main menu item. Also notification items in xoops_version.php are read by notification functions, comment items by comment functions.
You cannot rely on xoops_version.php to execute code for you on every page, if that is what you want.