I believe I've found a bug.
First of all I understand that on levels without a "target" to kill a "silent assassin" rating is not possible, therefore the highest possible rating would be "shadow."
Second, searching the net, I know that a Silent Assassin rating is possible on the Hunter and Hunted level.
As I playthrough the level, non-targets are subdued (not killed) and their bodies are always hidden. Targets are killed "signature style" for the highest possible points, and evidence is always removed. I'm never spotted, as I'm a veteran Hitman player & I study all npc movement patterns. Finally, all various challenges have been completed. What gives?
I searched google and the Hitman forums only to discover a bunch of pissed off people with the same issue. Ps3 players have not yet complained (to my knowledge) but both PC & 360 players have. Anyone know anything about this??