Switch between mobi site and full site

Hi guys.  I've noticed that on the demo NOP 2.4 store, I can access it on my iPhone 4 and I get the mobi site.  There's a link at the bottom to switch to full site.  After switching to full site, there's a link at the bottom of the page to switch back to "Mobi site".

But, on my own 2.4 installations, I only have the "Switch to full site" at the bottom of the mobi site, but no "switch to mobi" on the full site.  Why is that link missing from my full site, and is there a way of adding the link manually to the full site?  Right now, once you've switched to full site, there's no way for user to get back to my mobi site.

Maybe you're using a custom theme?
https://www.nopcommerce.com/boards/t/14995/view-mobile-site.aspx