The Shadow Tracer by Meg Gardiner

From award-winning author Meg Gardiner, co-author of Michael Mann's Heat 2
Sarah Keller is a skip tracer, tracking criminals who have gone on the lam. She's also a single mother to five-year-old Zoe, living quietly in small-town Oklahoma—until an accident sends Zoe to the ER, and their life changes in a heartbeat. Medical tests reveal a desperate secret Sarah has been hiding for years: Zoe is not her daughter, but rather a girl once rescued from a nightmare of murder, vengeance, and harrowing family ties she can't possibly remember. Sarah does. And someone wants to make sure she never forgets.
Now Sarah must abandon her carefully constructed life and take Zoe on the run. Using her knowledge as a skip tracer to stay off the grid, she must remain one step ahead of her pursuers if she is to stay alive, save Zoe, and bury the past once and for all.