The best option for me is the list of directories.
The flag inside file may also contain the name of currently logged Windows user, so if it doesn't match autosave disabled. Some kind of alternative is to check file creation and modification time - the first one should be greater if you saved the file at least once from MPT, and lesser if you copied the file to different path. If the file is copied to a friends in archive the archiver will set file's original date and time (WinRar does), so you should copy the file to another place before archiving it. That method will also work on all file formats

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The differential autosave will be headache definitely :wink: (at least until MPT code will contain general undo mechanism).