Can you complete the Cayo Perico heist without activating the alarm?
Spoiler I guess, if you haven't completed it.
So far I've been able to get just outside the lift in the main compound. Having only the mega armored guard and one last guard. Every time I kill the very last normal guard, the alarm sounds. Is it just coincidence every time, or will the alarm sound always?
It is most definitely possible to fully stealth the Cayo Perico heist, as seen in this video. Most likely the reason you are triggering the alarm is that there is a camera out that is not visible on your minimap that has the guard in its vision.