He assumes it, I don't think he knows definitively that we have it, but he seems sure that the people from the ship they're looking for do have it. I think even if he reads your mind he only knows we were on the ship, but I haven't done that scene after the artifact bridge scene.
Maybe we're all jumping the gun here, and Voss is a double-agent doing what he can to keep the weapon out of Vlaakith's hands...or maybe it'll be retconned that way later.
Lae'zel makes a lot out of following protocol and the Kithrak breaking it, but it's difficult not to see her as deluding herself there, I wonder.
