S2·E89 Play the Thieves' Symphony

Within the special vault of the World Crime Prevention Devices Association are drawings of all the world's safes. It's impregnable, with explosives rigged to go off at the slightest change in weight in its elevator, and a sound-gathering and detection system connected to a nuclear reactor. Lupin even retires to the mountains and devises a sound-dampening device, but is defeated by the sound of the device's own functioning. Surprise of surprises, what is the special vault's blind spot?