If the enemy cuts the cord, the loop of wire in the cord will be cut, and hence the computer will know something has happened.

(i.e. you have a lanyard with a magsafe-like connector with two pins. There's a resistance wire that runs inside the lanyard from the connector, up through the loop, and back to the connector. The computer checks that the resistance remains the same.)

If you want to get fancy, you can embed a RC network in the lanyard and have the computer sweep frequencies measuring reactance.