Mycroft had known Sebastian had something to do with the creature; the handwriting on the glass, done in black marker, is his (though it's not in a language Mycroft knows [interesting]). That the wizard is able to create a semi-aware being out of only light, however childish he may think it, is more than enough to impress a baseline human from a non-magical London.

“Indeed,” Mycroft says in bemused agreement.

The light Beatrice gives off from inside the glass is soft and pleasant—he can see why Irene enjoys it, despite it being in the form of an animal—and he admires it for a moment before speaking again.

“I confess, Mr. LeMat, that I was surprised to hear of your close acquaintanceship with Ms. Adler. You must have become friends quite quickly.”