In order to achieve the rank of silent assassin, you MUST:
1) Only kill the targets in each mission; you can however kill regular
guards or anyone else in an accident which includes pushing down a flight
of stairs or over a balcony.
2) Retrieve surveillance tapes if you have been caught on camera.
3) Make sure there are no witnesses or this will compromise your rank.
You will know when there is a witness as the notoriety meter will turn
yellow when you are near certain people. In this case, restart or reload.
4) Make sure that no bodies are discovered, unless it is the direct
result of an accident.
you CAN:
1) Fire as many shots as you want in a mission, provided nobody hears the
gunshots.
2) Use the fibre wire, syringe and accidents to eliminate your hits.
3) leave behind your suit and escape in disguise.
4) Let the notoriety meter go up to yellow, provided it is not because you
have gotten a witness.
You MUST NOT:
1) Use unsilenced weapons to eliminate targets, otherwise this will mean
you will score points for noise. You'll lose your ranking even if you
achieve all zeros on everything else.