This is an extension written by Adel-drago01-Gadllah (GNOME dev), that looks periodically for Extensions updates and if any it pop-ups a Shell notification.
The extension is seeking for updates 3 mins after you enable it, and every 12hours since then. It is quite easy to change these values from source code, or even add a notification that shows what extensions need updates.
A pretty cool extension that should be on Shell by default!
Extension Update Notifier is available for GNOME 3.10 and 3.12
Remember that there is an easy way to remove extension compatibility check. That is only available on GNOME 3.12