They made a big deal that the tape recorder was taken from her as she entered the fortress place that's why I thought it was something supernatural that made it appear in her pocket, but I can't figure out who had the right type of power to so something like that, my best guess is Amber was able to reverse time in a confined space and brought Kirihara's pocket back to a time when it contained the tape recorder. This wold also sort of explain the big guy's vision of the flame girl. Amber could have moved his brain from when he died backward in time so he knew it was coming.