It's dust. I've seen too many pics taken with the nav cams to think otherwise.
The "UFO" on the LEFT is in almost every nav-cam image taken from the left side. If the pic from the right side gets put online, you'll see both UFO's are missing. They are taken simultaneously.
Just look at the landscape shots from the day (sol) before, you'll see what I mean aboout the UFO on the upper left...
Quote from that dude: "This is not "dust on the lens" or "something on the film" as the usual idiots will chorus. This is a high resolution digital image"
He's gonna feel like an idiot when the right side nav pic gets put online. I'll gladly eat my words if it shows up on the other one though.