To accomplish this you will need to use javascript and only trigger it if the cart contain items.


Take a look at

- [Popup box on page exit][1]

- [How to show pop-up survey with Jquery when leaving page][2]

- [Magento How to check if the shopping cart is empty or not?][3]


  [1]: https://stackoverflow.com/questions/12557627/popup-box-on-page-exit
  [2]: https://stackoverflow.com/questions/8118722/how-to-show-pop-up-survey-with-jquery-when-leaving-page
  [3]: https://stackoverflow.com/questions/12779110/magento-how-to-check-if-the-shopping-cart-is-empty-or-not