Former Nirvana bandmates, Dave Grohl and Krist Novoselic, recently recorded with Butch Vig, the producer of the Nevermind album. The recording is likely a part of Grohl's Sound City Studios project. Although, Novoselic and Vig did contribute to Foo Fighters' Wasting Light album.
Vig tweeted, "The last 24 hours have been surreal! Had a great gig at the El Rey last night, and spent today recording with Dave, Krist and special guest!" Who is the special guest? Grohl is likely on drums (he rarely sings and plays guitar outside of Foo Fighters) and Novoselic is likely on bass, so guess a guitarist or singer.
Sound City Studios is the recording site of several famous albums, including Nevermind. Fleetwood Mac's Rumors, Neil Young's After the Gold Rush, Tom Petty's Damn the Torpedos, and Rage Against the Machine's self-titled album are among the many records recorded at the famous studio.