UPDATE: ONI's been spreading misinformation again. The official Halo Twitter account has issued an apology for the ODST remaster listing, saying that it was an erroneous placeholder that was put out too soon. Unless the add-on comes within a few days, it looks as though 343's hopes for a May release weren't quite met:
A placeholder release date for the ODST campaign for Halo: The Master Chief Collection was inadvertently listed on the Xbox Store. (1/2)May 27, 2015
We continue final testing and will make an official announcement once our release date is determined, and apologize for any confusion. (2/2)May 27, 2015
Original story: Well, 343 Industries did say that the Halo 3: ODST remaster, soon to be available for free to anyone who played Halo: The Master Chief Collection before 19th December last year, should be out this month. That was looking like an increasingly shaky proposition until some delightful doofus put up a listing for the DLC on the Xbox Store, along with a release date of 29th May (i.e. this Friday).
The add-on appears to be 8.1GB in size, and promises the usual 1080p/60fps experience. The listing's since been removed - I'm guessing it was meant to be a drop shock of its own - but it seems more than likely we'll be playing Bungie's non-Spartan side-project come the weekend.
There's been no word as yet on the remake of Halo 2's Relic multiplayer map that seemed to be coming alongside ODST, although - and I am possibly showing my limited understanding of development here - I'm guessing remastering an entire game's worth of content would be a more intensive process than a single island with a big tower in the middle.
Now we just need Reach, eh?