Every alarm in the building goes off at once. Fire, tornado, civil defense, security breach, power loss, and any number of others. Emergency lighting and sprinklers turn on. Halon systems dump throughout the computer complexes. In a word, pandemonium strikes.
Doors go into emergency evacuation mode, sliding to a locked-open position on battery power.
A stampede begins. The intruder waits at the edge of the main driveway. The complex begins to belch herds, nay, hordes of giraffes. As each runs blindly through an exit, its head is cleanly sliced from its body by the undetectable thread. Soon the lot is littered with thrashing giraffe necks, bellowing unnaturally, eyes rolling in frenzy or staring in hatred.