If one of your groupmates could sneak (including autosneak) then they may have swiped the weapon without your noticing. A ranger or assassin in the area could also have been pulled it off.

Did you locate the weapons after they disappeared?

Another possibility is that your weapons were over limit and poofed when the archer died (like they would if you had died while carrying them). I'm not sure if the code works this way, but I wouldn't be surprised if it does.