Multiple Reddit users (by way of Polygon) are reporting that they have happened to stumble upon something pretty interesting on Xbox One's marketplace. Seen on the Xbox One Marketplace was a listing for a "Watch_Dogs Closed Beta".
The 23.07GB beta was not accessible for downloading, but the listing was found through the Upload app and a filter by game name. One in the store, Reddit user AbleToFail found the listing for the beta. Reddit user Mouse2012 found a link to the beta as well but by different means, and there was still no way to actually download the content.
Polygon reports that the "Watch_Dogs Closed Beta" listing is no longer accessible and that they've reached out to Ubisoft to get some info. Internal betas are probably fairly regular, and maybe this build was for dev units or an eventual press preview. Either way, don't go about frantically searching for the beta in hopes of playing the game early. The listing was probably just mislabeled and accidentally made visible to the public with no real means of accessing the content.
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Similar listings have appeared in the past on Xbox 360. I remember the days when the console was still young and Microsoft would accidentally place items for free on the store during the early hours of the day, only to fix them when someone noticed. If I remember correctly, I think I was able to nab a free Uno skin pack and maybe some DLC for a game; it was so long ago. But point is, mistakes will happen and we'll sometimes see behind the curtain.