He might have been able to easily follow almost anyone else. But Ruby was a demon, and moreover she was a pissed off demon which meant that she was on alert pretty much all the time, and she knew when she was being followed.
Her grip tightened on the knife and she whirled around, not looking halfway amused by this. "Come out, now." She ordered. She wasn't in the mood for games. She didn't know what was following her, but something definitely was.