In OS X, if you rename or move a file, whether from Finder or even the ST2 sidebar, the open tab for that file does not reflect the change. Clicking that tab, editing, and saving will then create a new file with the original name/path.
OS X applications like TextEdit will notice that the open file has a new name and/or path and will save to the new place.
What's particularly confusing is the sidebar itself does detect updates. If I change my project layout a bit in the Finder, the sidebar will notice and reflect that immediately. However, the open tabs still refer to the old layout and I'll have to close all the tabs and re-open.