How does rockstar prevent any leaks?

A 1,000-person team developed GTA 5, so how does rockstar manage to prevent ANY leaks on GTA 6?

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Consider they've been working on the game since 2013-2014. Not a single employee, from a team of 1000+, has said a word about the game in 5 years.

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Obviously some employees have left rockstar over this 5 year period, so why wouldnt any former employees (anonymously) leak details on the most anticipated game in history?

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I understand that they compartmentalise the development of the game. But still, if a graphic artist is working on a map, they'll obviously know where the map is based off, even if they aren't told any other details about the game.

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The only reason I can think of, is Rockstar monitors employees communications like the CIA, and employees have signed an NDA stating they'll be sued to oblivion if they were to leak details.