The last we see of Kate is her sitting in the tractor alone near the crash site. Carter had earlier told her there was a Russian outpost nearby (though it's odd he didn't mention Outpost 31...). Did she make it there in the tractor? Did she only manage to get part way to the base, but was found and rescued by the Russians in time? Or did she die out there in the tractor? Or, even more chillingly, did the Russians find her and bring her back to their base....but she was a Thing herself? I think it's unlikely she was assimilated, as there were no live Things in the area at the end, but I;d like to hear other people's thoughts.