weird mouse coordinates on site

Question

I've added logging of mouse move events on my site, just tracking any mousemove X and Y:

window.onmousemove=function(e){
   mouseMoves.push(e.x)
   mouseMoves.push(e.y)
   );
}

I do this relative to the window element, or the document for old IE browsers. It seems to be working well, except occasionally I will see coordinates where the Y value will be very high - 3500 for example.

How is that even possible? are there screens out there that can show 3500 pixels of my site, or does the user somehow move his mouse to a part of the screen that's not visible.. or are these bots?

I'd appreciate any insight into how this is happening..