Mycroft recently thought of Ilde as having the impetuousness of youth and the trained falseness of age, so they're matched here in their contradictions and their goofy shared delight. It makes sense, then, that all of their secrets are going to be--

Penelope-- (hereafter referred to with a fisted gesture that is, actually, kind of obscene) --stitches secret messages to the Illuminati into the Vault costumes.

...like that.