New Post: Code added at the end of all URLs

Pages of our composite website get a random code added to the end, e.g.:

http://www.mydomain.com/mypage#.UqsdsNJdWbM

It seems to carry across all pages per session. So once you start browsing the website all pages get the same code added. It happens a split second after the page is loaded.

Any ideas what this is and how I can get rid of it?

Thanks in advance