I'm writing a novel, in which one of my characters is kidnapped. Fortunately for her, her kidnappers aren't extremely good at the whole kidnapping thing. She manages to gain access to one of their phones, and also finds an opportunity to escape.

Here's her predicament. For various reasons (plot stuff :-), she doesn't want to call the cops just yet. But she DOES want to get away, and she DOES want to track her kidnappers so that she knows where they go next. Is there a way for her to install some kind of app or something in the kidnapper's phone that can enable her to track it undetected? I know there are spy apps out there, but not sure if it's feasible to download anything in her situation.