experienced another "untargettable target" case in mansion map, this time line of sight was 100% unobstructed but I just couldn't target the alien. Sooo... I spammed nades and full auto plasma fire in it's general direction and heard some pain sounds, so far so good but apparently it had become a zombie! for the following rounds it just sqatted there, detectable as a visible target by pressing t, doing nothing but staring the kerrblade in front of him. Curiously, he also had one of those in his hands... Plasma fire whizzed right through him though, so maybe it was a ghost after all and not a zombie. And, when none of my soldiers were looking at him for one round and then turned back towards him, gasp, he was gone. Just the abandoned kerrblade lying on the ground, lone and forgotten. Spooky!
(in the "original untargettable target case", the alien died normally. as far as I can tell what is normal for an alien, but anyway.)