The thing about announcements is that before they are announced they are by definition secret. That's also generally how planning the future of a business works. Chances are this is just a tongue-in-cheek reference to Part 1 of Musk's Tesla plan, which was outlined way back in 2006 on the company blog. The important part of that rambling missive is as follows:
Musk has also said, via Twitter, that there will be a second part to the Model 3 announcement. Apparently he likes doing things in two steps. Perhaps this second part of the Masterplan includes the Model 3 news, perhaps it doesn't. What exactly Musk and Tesla announce this week (or maybe next...) will be one of those secret surprises companies usually call making news. We'll just call this a heads up.