It is most definitely a temporary file used and created by the kobotouchextended driver plugin for calibre.
https://github.com/jgoguen/calibre-k...driver.py#L262
You're not going to see the file unless you open the epub as a zip archive. It is my understanding, however, that the plugin does not modify an epub in calibre's library. It updates epubs when they are sent to a kobo device. To support "extra" kobo features. If that's true, you must be using eboubs that were a) delivered to your Kobo using the KoboTouchExtended driver, and then b) subsequently pulled off of your Kobo before editing with Sigil.