On this page of the guide, you will find an explanation of the secret ending of Hitman 3, which can be unlocked during the Untouchable mission on the Carpathian Mountains (Romania) map.
If you pick up the syringe after meeting Arthur Edwards (The Constant) and wait about 1 minute, you can unlock an additional option in which Agent 47 injects himself with the serum that will make him lose his memory once again.
When Agent 47 loses consciousness, Arthur Edwards will say that he will take care of the main character. After a while, the titular Hitman will wake up in a padded cell with a massive iron door. You will hear Arthur Edwards' (The Constant) voice from the speakers.
Long-time fans may remember this scene from the first installment of Hitman: Codename 47, which was released in 2000. So it can be argued that by choosing this path, Agent 47 has come full circle and has once again become an assassin with amnesia.