Currently, to find the easter egg in question you have to activate the cheat first.
Ubisoft and Square Enix are collaborating to present the nuanced Assassin's Creed event in Final Fantasy XV. This event has been going on since last August and will end January next year.
Recently, a gamer found an easter egg in Assassin's Creed Origins suggesting that the collaboration between Ubisoft and Square Enix is not just to present the event in Final Fantasy, but still continues. However, now turn game from Ubisoft which will get a little touch nuanced Final Fantasy.
Easter egg in question is a temple complex where there are pillars with holes on it. The holes are likely to be used to complete the puzzle in order to enter the main temple.
Inside the main temple, there is a wall with a certain pattern image, and the image resembles a Square Enix logo with XV writing underneath. Then go deeper there is an empty space that is likely to be used to place certain prizes.
However, keep in mind that for now the area was not accessible directly. Because it is still not finished alias in the stage of workmanship. To access it, must use cheat free camera first. For location, can be seen in video.