Mycroft notices the note directed at him, and raises an eyebrow at the nearby mug (which features a ceramic interpretation of the back-end of a gun as the handle). It has a bit of money in it, though not much. Unwilling to spend his limited funds on something he's not sure of, Mycroft instead decides to venture inside the room.

He hadn't noticed the girl looking back and seeing him. How had she known he was there? Some sort of power, then, like Charles has? With the possibilities so endless in this place, huge blanks have opened up in Mycroft's observations.

It's simultaneously horrifying and thrilling, like being blindfolded and feeling your other senses heighten in response. The fierce joy of learning something new at last has been burning in his mind ever since this morning; the presence of others in the library, of so many interruptions, has kept that burn low, but oh, it is there. This young lady and her diagram are just adding fuel, his pull toward them not so much an interruption as the opening of another book, the way his conversation with Charles had been.

"Which system is this?" he asks.