Hidden achievements:

Impostor achievements:
[list:]
Circumventer: Win as impostor without using a vent
HUNGER: Kill 3 crewmates before the next meeting (no matter if at round start or in between meetings)
Never suspect a thing: Complete a 2 impostor game with both impostors alive at round end
Crewpostor: Fix a sabotage you caused
Scourge: 150 kills as an impostor (must be normal game mode -> hide and seek doesn't count)
Crewmate achievements:
Watch me scan: Die during a medbay scan (you can just hop on a scan and cancel it all the time until the impostor comes around)
Impossible Task: Complete the card swipe (on first try!)
Taskmaster: finish 500 tasks