This seems to work well without scripts, I've used it myself.
Trigger
Activation - East Present (if that's the enemy burning the files)
Timeout - min 5, mid 10, max 15
Condition - {behaviour _x == "COMBAT" AND alive _x} count thislist > 0 AND isServer
On activation - alarm = true; publicVariable "alarm"
NOTE: isServer and publicVariable is only needed in Multiplayer. Also, this only works if you kill ALL group members within the random timeout. Meaning: A single guard is easy, a 10 man group is more or less impossible. You can always increase the timeout numbers.
Hope that helps,
Laggy