The rumor floating around NeoGAF is that a new Rainbow Six entry is in the works, as spotted by NeoGAF'er wwM0nkey. Said GAF user posted a tidbit from recording engineer Sean Popejoy's LinkedIn profile, which states he "developed Pro Tools sessions from Ubisoft audio for the game demo of Rainbow Six (2011)."
wwM0nkey also posted the domain name www.rainbowsix-BD.com, which is owned by Ubisoft.
Hopefully, we'll hear something soon...as in, E3 soon.
UPDATE: It appears the Rainbow Six segment of Popejoy's LinkedIn profile has been taken down, but information about the company Tonehammer is still up and indeed mentions "video game composers" as one of their specializations; obviously, this adds to the mystery.
Original story via Eurogamer
