Cleaning up sessions in mongodb, expressjs, nodejs

My configuration:

app.configure(function(){
    app.set('views', __dirname + '/views');
    app.set('view engine', 'jade');
    app.use(express.bodyParser());
    app.use(express.cookieParser());
    app.use(express.session({
        secret: 'MY SECRET',
        store: new MongoStore({
            db: 'MY SESSION DB',
            host: 'localhost',
            port:88888
        })
    }));
    app.use(everyauth.middleware());
    app.use(express.methodOverride());

    app.use(app.router);
});

app.configure('dev', function(){
    app.use(express.errorHandler({ dumpExceptions: true, showStack: true }));
    appPort = config.port; //Setting PORT to 8888 in dev mode.
    app.use('/public', express.static(__dirname + '/public'));
});

app.configure('production', function(){
    app.use(express.errorHandler());
    appPort = config.port;
    //Set cache-header-expires to 1 day
    var oneDay = 86400000;
    //app.use('/public', express.static(__dirname + '/public'));
    app.use('/public',express.static(__dirname + '/public', { maxAge: oneDay }));
});

Now I have a "logout" link that goes to / logout in my application.

AFAIK, express automatically performs cleanup sessions upon logout. But with my configuration, I do not think so. For example, a user variable attached to a session

req.session.custom

saved after logging out. Nonetheless,

req.session.auth

cleared after logging out.

The number of session objects in my MongoDb repository only increases over time. I use everyauth .

What am I missing or is something wrong?