Ginny didn't know how she returned home, she felt like she was still in a daze. In fact, she was in a daze the second she found out that the Terranova family was allowed to use the truth serum. It was safe to say that the use of the truth serum was highly restricted, even the Royal family had to write a detailed report to the high priest before they could use the truth serum.
The law was passed down ages ago or else people will use it willy nilly and that was why the high priest had the right to validate the use of the truth serum as per the requirements.
But from what Sir Fox said they can use the truth serum when they want, how they want and without asking for any permission.
