Working on Top Secret Tesla Masterplan, Part 2. Hoping to publish later this week.— Elon Musk (@elonmusk) July 10, 2016
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:
Build sports car
Use that money to build an affordable car
Use that money to build an even more affordable car
While doing above, also provide zero emission electric power generation options
Don't tell anyone.
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.