We are more than halfway through the year and the next installment in the incessant "Call of Duty" series is still a complete mystery. Activision hasn't revealed anything, and so far we've only been relying on leaks and rumors. But now it may have appeared on the Microsoft Store.

A listing for an Activision shooter called The Red Door recently appeared in the store without fanfare or announcement from the publisher.

"There is no one truth. If you go looking for answers, be prepared to question everything and accept that no two are the same. A red door awaits you.

This listing is intentionally mysterious, so it does not appear to be another case of a game appearing in the store ahead of schedule. Since it has not yet been removed, it appears that Activision is involved in this case and is teasing an upcoming game.

But where is the Call of Duty connection? The rating, genre, and publisher all apply, but there is no clear link to the series. However, this is not the first time we have seen the name "The Red Door". Last month, the PlayStation serial database SerialStation listed the game, along with a content ID that sheds light on this mystery game: UP0002-CUSA20046_00-COD2020INTALPHA1.

The listing on the Microsoft Store seems to confirm that an alpha version of Call of Duty 2020 is on the way.

The listing does not give any details about the setting or final name, but it is rumored to be "Call of Duty: Black Ops - Cold War". This was leaked in May, but the Cold War setting was suggested by Kotaku last year. The game is said to be set in Vietnam, and was initially being developed by Raven and Sledgehammer, but is now being handled by Treyarch.

As noted on Reddit when the SerialStation listing was discovered, The Red Door already has ties to the Black Ops series; The Rolling Stones' Paint it Black was the Black Ops 3 reveal It is used in the trailer and begins with "I see a red door and I want it painted black." The song also appears in the credits of Full Metal Jacket and was recorded during the Vietnam War.

Evidence seems to be piling up, but Activision has yet to say anything. No date is given in the store, but more details are likely to emerge in the near future.