Sharon
Solely by process of elimination, I'd guess Dr Shaw and Dr Aphra are the least unlikely, but that's really not saying much.
I really try not to let things like that too close to me. You'd probably get a lot more out of the vibranium in my uniform than by selling my organs on the black market though, if you were after money and weren't too worried about an angry cat guy chasing you down.
Our main options are probably 1) It was someone onboard, either accidentally or for an unknown reason 2) Revenge from the Sirenians or whatever they're called, depending how far away our route's taken us 3) An unknown group targeting something or someone specific, which really could be anything 4) An unknown group that just thinks we look mighty tempting