It sounds as if Grand Theft Auto V is about to get the "Undead Nightmare" treatment.
YouTuber Chr0m3xMoDz has claimed to have uncovered a downloadable content selection screen in GTA V's code. One of the options is labelled "Dead", and when selected, it checks for "SPZOMBIES".
Elsewhere in the game's code, players have found "ZMB" strings.
There's precedent for a Rockstar-developed zombie mode. In 2011, the comapny released Undead Nightmare for Red Dead Redemption, an add-on that saw protagonist John Marston fight a zombie plague on the Western frontier.
Check out Chr0m3xMoDz's claims for yourself.