Internet sleuths digging through game code from Grand Theft Auto V purport to having discovered several signs that suggest that zombies could be a theme for a future downloadable content expansion (thanks VG247).

Notable GTA modding youtuber Chr0m3xMoDz has been sharing bits and pieces gleamed from decompiling bits of GTAV game code for some time, and recently shared what is supposedly a placeholder DLC selection screen, wherein one of the options is "DEAD" and when selected, attempts to launch SPZOMBIES. Other references to ZMB were also found elsewhere.

Developer Rockstar did the zombies schtick previously in Undead Nightmare, a reasonably popular content expansion for Red Dead Redemption, so it's certainly plausible. That said, plenty of games are littered with code fragments for unfinished and cancelled features, so despite the excited "100% confirmed" titles on Chr0m3xMoDz's youtube videos, we really won't know for sure until we hear it from Rockstar itself.