When you steal the primary target, 5 new guards spawn. The one that appears near the left wall causes the alarm: if the previous guard there is killed (remember you only have to kill him if you already killed all the others except the two guards at the main gate - if the dead ones did not drop a keycard; and also kill him when he is standing next to the wall). After you grab the main loot, you need to kill the new guard immediately - one player should be there and be prepared to kill him. Again, do not kill the guard going round the main gate and the one up above the main gate.
When I do it with my friend, they get an alarm the moment we steal the main loot, I couldn't understand why.
When you steal the primary target, 5 new guards spawn. The one that appears near the left wall causes the alarm: if the previous guard there is killed (remember you only have to kill him if you already killed all the others except the two guards at the main gate - if the dead ones did not drop a keycard; and also kill him when he is standing next to the wall). After you grab the main loot, you need to kill the new guard immediately - one player should be there and be prepared to kill him. Again, do not kill the guard going round the main gate and the one up above the main gate.