OK, got through Countdown with two non target NPCs subdued and hidden. My score was zeroed before Dexter. Signature killed him, but I only got Professional rating. Are you kidding me? Is it even possible to get Silent Assassin on Countdown? Some claim they have, but I've yet to see anyone prove it.
I was real close to sneaking by the first bunch after shooting one of the bombs as a distraction, but then I got caught after grabbing the hammer and every reload resulted in the AI taking different paths after that when shooting that same TNT pack. It's easy to sneak by the second bunch after the checkpoint with a moderate amount of Instinct, but at this point I'm not sure even sneaking without any pacification would get me a better score.
Maybe you have to accident kill Dexter to get Silent Assassin.
I'm on Epilogue now and I had to take a break because the game has got yet another odd thing about it. I got by several of the first guys unseen, then found a good spot to hold up until the coast is clear, but it involves pacifying and hiding an NPC. I get him subdued, take his disguise, wait patiently behind cover while a nearby NPC stands looking my direction, then when I go to hide his body in the grave it counts as killing him.
Again, are you friggin kidding me? A 6' grave with a coffin in it is all of a 4' drop, and that's going to kill a subdued NPC? I've seen places in the game where NPCs survive twice that high of a drop when you hide them. Just can't stand this total inconsistency in the way the game plays. Then again, maybe this is one of many unintended bugs.