Bem, um grande ponto deste filme é que
Miranda Tate is actually Talia al Ghul, the daughter of Ra's al Ghul, and she has been working with Bane all along.
Então, desse ponto de vista,
she never had to get "free" from Bane.
Parece que
she and Bane decided that Lucius could go free, and that she would tag along with Lucius. To explain to Lucius why he was letting them go, Bane most likely told them that they could do little to harm his siege of the city.
Isso seria vantajoso porque
Lucius would probably seek out or organize some form of resistance, and so if anyone was planning to usurp control of Gotham from Bane, Lucius would know about it or be involved. By sticking close to Lucius, Miranda would be able to inform Bane, who could then mount a defence. Basically, a free Lucius was more valuable to them than a captive Lucius.
Isso é exatamente o que aconteceu,
given how quickly Bane had found the Special Forces men after they had been brought to Lucius and Miranda.