Update: Halo 3: ODST's May 29 release date was a placeholder and not the final date, the official Halo Twitter account has announced.
"A placeholder release date for the ODST campaign for Halo: The Master Chief Collection was inadvertently listed on the Xbox Store," reads the statement.
"We continue final testing and will make an official announcement once our release date is determined, and apologize for any confusion."
Original report: Halo 3: ODST will launch for Halo: The Master Collection on May 29, according to listings spotted on Xbox.com and inside the Xbox store.
The game is being offered as a free download to all Master Chief Collection players who bought and played the game between launch and December 19 - the period in which the game was deemed to be at its unplayable worst.
There's no word on if and at what price the game will be released to those who don't meet the freebie criteria.
ODST will run in 1080p at 60 frames per second.
Halo Reach will be the only remaining Halo FPS not included in the collection.