In some cases, the "Debug Memory Graph" feature in Xcode shows problems that have no references -- just a single NSURL or CFString. I can see the allocation backtrace, but I'm not sure what Xcode is trying to tell me about that object -- why is it a problem? Doesn't seem to be a cycle. Is it leaked in some other way? There's no text explaining why that object appears.
This looks like a bug in the memory graph debugger. It could be that the object has a reference to itself, but that should appear on the graph as a circular arrow. Alternatively, the graph is somehow missing a reference that is keeping the object in memory.
Please file a report at bugreport.apple.com, and if you can, export the graph (from the file menu in Xcode) and attach it and the Xcode project, along with a description of how you got the app in this state.