Hello Duncan Evans,
Strange enough, I do not see the broken link either.
I spent some time with the error, and figured out that this is a problem in Sitefinity 3.6SP2. When you have a link to a secured page and you are logged out, you get a link like:
site.com/item not found: [provider]Guid
This is resolved in 3.7 though, so if you see any errors in the log related to that problem, you could consider an upgrade.
the Telerik team