Talia believes Bruce to be captured. She doesn't know Catwoman is going to bust him out so he's long gone doing whatever before the trials start. Bane doesn't ask for Miranda until after the trials end but we don't know when exactly they meet up. Regardless Talia still doesn't know that Bruce was set free by Catwoman. She knew he was planning something but didn't know what, when, or how so even if she had met up with Bane to tell him that Bruce was back she still doesn't know what's happened with him. After the symbol is lit it's a merc that draws Bane's attention to it asking him do you think it's really him and Bane replys "impossible." That would lead me to believe he hasn't met up with Talia yet and he didn't know. Even if you believe that Bane and Talia had already been reunited the impossible line could still work in the sense that he didn't know Bruce had escaped because Talia didn't even know Bruce had escaped.