Apologies, I have not updated the site correctly yet, I am in pain today, I'm afraid.

As per your suggestion, I used the following code to no avail. It did not delete the cookie. Nor did it send me an email as requested, which shows me it not get to the error log. I note that using the option '3' in the error log request did not write a log file either (if I put that at the top of the function!).

function recordlogout_init(){
        ossn_register_callback('action', 'load', 'recordlogout_cb');
}
function recordlogout_cb($callback, $type, $params){
    if($params['action'] == 'user/logout'){
        $cookie_name = "OSSNuser";
        setcookie($cookie_name, time() - 3600);
        error_log("logout",1,"[email protected]");
        return true;
    }
}
ossn_register_callback('ossn', 'init', 'recordlogout_init');

Thank you for quick response

There is no way to find when user is logged out by system due to inactivity. If you are looking when users logs out themselves using button,

function my_component_init(){
        ossn_register_callback('action', 'load', 'my_component_cb');
}
function my_component_cb($callback, $type, $params){
        if($params['action'] == 'user/logout'){
            //do some stuff......    
        }
}
ossn_register_callback('ossn', 'init', 'my_component_init');