Fire event when mouse is over nothing?

How can I make an event fire when the mouse is not currently hovering over any object?
I have a script which detects when the mouse is over objects and which objects (entity picking), which highlights squares on a grid, but when the mouse leaves the grid entirely I want to remove the last highlighted square, currently it just stays highlighted even if the mouse moves to a different window for example.

Here is the mouse hover code (inside an initialize function):

    app.mouse.on(pc.EVENT_MOUSEMOVE, function(e) {
        var from =, e.y,;
        var to =, e.y,;, to, function (result) {
  'EVENT_HOVER', result.entity); 
    }, this);