A new version of Britney Spears' song Passenger was written by Sia (and is also credited to Spears, Katy Perry, Thomas Pentz, and Andrew Swanson) and appears on the album Britney Jean. Here's the album version, and below is an acoustic version that popped up online. Could it be a demo? It's surprising in that it's a duet with Sia, it's acoustic, and Britney sounds great. The guitars are played here by Nick* & Country Club Martini Crew.