This is actually displayed on the USCIS website under your case files:
* Note on "Last Updated" date:
Your case may have a new "Last Updated" date and you may receive an Email Notification of your case being updated, without the status of the case changing. This is due to internal USCIS processing being performed on your Case. This will be reflected in the "Last Updated" date, but may not result in a different status message.
Take it as a good omen that someone has at least looked at your file, but try not to get too excited until they actually change the text. I know its hard, we all get excited when we see LUD's. But it will drive you crazy wondering what exactly is going on until they actually inform you.