Yes threaten legal action as said , but tell her that you are going to the police right now to report that the keys have been stolen by her from you and the other exec.
I think you are going to have to call her bluff and play a bit dirty in this case.
Absolutely disgraceful in my view.
The police may not want to know initially , but if you make a big song and dance about it they will listen , especially if you casually mention that you believe she may be about to empty the house and sell off the contents.