All that would be needed is a way to ensure that the gravity systems would be affected when the torpedo struck. A simple slave circuit and an arrangement so that the circuit evidence would be hidden by further damage. The investigation was so poorly done that they could not even determine the source of the torpedoes was not the Enterprise, even with the fact the ship was still fully loaded and there was no record of the launcher being used. If they could not find those two obvious facts which were known immediately after the incident, I doubt they could discover anything on an enemy flagship after damage control repaired the damaged systems...
