olark('api.box.onHide', function() {
	// Your callback function
});

Notes

Whenever the chatbox is hidden, the given callback function will be triggered.

Screenshot of API call

Notify an operator when a visitor closes the chat box

You can notify your operator that the visitor has closed the chatbox if you have already started chatting:

<script>
// Checks to see if the coversation has started
olark('api.chat.onBeginConversation', function(){

    // Triggers when the chat box has been hidden
    olark('api.box.onHide', function() {

        // Notifies the operator. The visitor does not see this.
        olark('api.chat.sendNotificationToOperator', {body: "Visitor hid the chatbox"})

    });

});
</script>

You could just as easily replace the onHide call, to be onShrink if you wanted to report to the operator when the visitors minimizes the chat widget.