Child’s was the thing at the end and Macready knew when he walked up.
Mac, Gary and Nauls go out to give Blair the blood test. Blair isn’t there and the door is open. Mac, Gary and Nauls go under the shed and find the flying saucer being built. But Blair isn’t down there, he is coming up to get Childs who is guarding the main entrance. We see a shot that seems to show the generator room downstairs, and the space where Childs was. Jackets and boots have been rearranged in that scene. Right after, Mac and Gary are preparing to drop dynamite down the shed hole. Nauls watches Childs exit the main entrance, but we don’t see Blair run out first. Blair is taking the generator down the stairs,and seconds later the power goes out. When Childs tells Macready he ran out because he thought he saw Blair, he’s lying. Macready knows he’s infected, because he believes what Nauls said, and knows Blair could have got to him.