OK so one of the game developers (Reza Elghazi) posted on twitter a response to someone who asked "is there more to the easteregg after the tower of babble achievement? perhaps a 2nd bus or in game reward?" he responded with a simple "yes". https://twitter.com/_rez_/
This video which is in Spanish shows a clue of the next step of how to how to get to through the next step of the easter egg.
According to a rough translation from a friend they did both maxis and richtoffen's easter eggs then they found this.
Let's figure this thing out! We can do this out post your findings or ideas, insights you might have on this.
There is proof in the PC gsc files that there is more to this Easter Egg without a DLC.
Thanks to solution4u for finding these links.
Here is an ENGLISH VIDEO about the lights and they got 2 different colors.