I dont remember how it worked in SP, but there are special script command:
SetAlarmType(512, 0);
Its alarm type 512, here its "0", it means its turned off for this enemy.
This one and some others dont work for clients in MP and they should be turned off, even if its coop mission, because if coop mission is started from game menu, it might make mission act different.
Strange thing is that even if "hear steps" is turned off, enemy still can hear steps, but dont get alerted (not for clients).
There are some ways fake this for clients...to make harder to kill enemy by sneaking from behind.
Ability to pick up and carry corpse cannot be turned on or off from scripts, this function seems to be off for MP from game functionality, even this part of menu is disabled...
You can fool around in sp by this corpse carrying ally, by commanding it to enter vehicle, somehow the corpse will get bugged and remain attached to ally and look very funny walking around with it.