Oh, and now--How shall I tell Gilbert?
How _shall_ I tell him?
What ought I to do, Mr.Ackroyd?
Thyrza made me promise faithful I wouldn't tell her secret.

She says that, in the note.

I'm sure she hasn't gone--gone to him.

She couldn't marry Gilbert, and yet she doesn't want him to lose the library.

That's why she's gone; I know it is.