You can hide them in various wardrobes or containers. No body can be left in sight at the end of the mission. It's risky, so it's best to avoid killing other characters completely. In the case of guards, you can arrange an accident - for example, drop a heavy load on the opponent. List of requirements for obtaining the Silent Assassin rank:
This is because you have to play these missions in a specified way - you can't avoid being detected, and you have to eliminate more people than just the targets. The game doesn't allow you to obtain Silent Assassin rank in the first mission (Death of a Showman) and in the epilogue (Requiem).
