Where did Betty hide Harriet?

From the text:

When I get you into missis' safe place, I will bring some nice hot supper.

The mistress came to meet us, and led me up stairs to a small room over her own sleeping apartment. "You will be safe here, Linda," said she; "I keep this room to store away things that are out of use. The girls are not accustomed to be sent to it, and they will not suspect any thing unless they hear some noise. I always keep it locked, and Betty shall take care of the key. But you must be very careful, for my sake as well as your own; and you must never tell my secret; for it would ruin me and my family.