Spoilers.....at which point do you think Captain and Sanya turned?

It makes no sense. They were fighting the turned ones all the way back to the ship, if they had turned on the way wouldn't tell have brought the others with them? Or can one body hold multiple aliens as they infer to Wess? Neither looks to have been shot so did the aliens perhaps gang up on the 2 with all their might? I suppose it was just as they approached the Argos after those other 2 covering their escape were killed.

You're a funny guy - but looks aren't everything.

My theory is while they were fighting near the Galiot they were overcome. Perhaps they will killed in away which didn't leave obvious marks on their bodies, and their bodies were then taken over.

So they pretended to retreat toward the ship with the other possesed ones petending to chase them.

And another possessed person was sent to circle around and shoot in the back the two persons defending their retreat.

Or another theory might be that Captain Sallis's dead body was foolishly left on the ship and not locked up. And Sallis might have later revived and maybe he killed the Captain and Sanya after they returned to the ship, allowing them to become possessed.