How does reddit update favicon on the fly without caching problems?

Question

Whenever I change the icon on the site I work for, it may take several hours / days for the browser to request a new resource again.

But when I go to reddit, if I have a message, I get a notification in the icon without loading the page, I think?

Is it as simple as supplying a new icon with a different file name? How can you load a browser without loading a page?