Backup of "untitled" file recoverable?
It depends on the application, but a program like TextEdit, that you mention, names unsaved documents as "Untitled xx" where xx is a number. You can then search for that name to recover the document from backups.

For example on many systems the untitled document would be saved in:

~/Library/Mobile Documents/com~apple~TextEdit/Documents/Untitled xx.rtf

If you have a Mac where the system has been shut down, and open windows have been auto saved, and you're looking at the disk from a different Mac for recovery, you'll find the auto saved window information in:

~/Library/Containers/com.apple.TextEdit/Data/Library/Saved Application State/com.apple.TextEdit.savedState

However, those files are not immediately readable as RTF files. The easiest way to restore them is to place them there on another Mac, and then start up TextEdit to restore them.