This is just speculation but in light of the new chapter. The strongest marine had an interesting comment to her.
Aokiji: Why didn't you just run away, as always? You could have escaped from CP9 at any time.
Robin: … Didn't I say things were different this time?
He said she could of escaped anytime? If that was the case, why did the strawhats have to spend all that time looking for a key is she could just escape.
Did he just mean during water 7 and on the sea train? I dont believe she was in cuffs than, but she still had 4 strong cp9 people there. Could she of restrained them and got away in the middle of the sea? Interesting.