Perhaps some scientific wizardry will do the trick. After all, what better method for a scientist to steal an engine?
The Argos Engine replica inside the glass case was made of a higly flammable material! Moreover, inside it was some highly volatile substance that was rigged to catch fire at the slightest impact.
The culprit waited for the blackout to happen, then kicked the stand to trigger the fire. The Argos replica was consumed in a burst of flame, which the culprit hid from view by covering the glass case with a cloth of some kind, such as a coat. The card had been encased in ice inside the replica, so the ice was melted leaving only it behind. Every other trace either burned away or fell into the small ventilation holes.