He was talking about Underline.

They had a spherical body with three wire-shaped cilia on the left and three on the right. Instead of walking on the ground, they floated through the air.

Those miniscule machines used the convection of the air to generate their own power, gathered data indefinitely, used a quantum signal they produced transmitted over a direct electron beam to pass information between them, and created a type of network. Underline was the sole direct line of information into the windowless building so those tiny bodies had to hold information from the darkest depths of the city that would shake the world if it got out.

“Even knowing of Underline’s existence, finding microscopic machines isn’t easy, and, even if you do capture some, there’s no way to get the information out. You have to open up a nano-sized device and attach a cord to it. Not to mention that apparently carelessly ‘observing’ the quantum signal inside it will cause the information to change.”

That was why the Tweezers were necessary.

No matter how small the nanodevices were, an item developed to grab elementary particles would have no problem. With it, getting the information out of Underline was quite possible.

Toaru Majutsu no Index vol.15 between the lines 4