Can Legal HeadHunters call the company you work for if they find out that your leaving if they recruit for company?

I just heard that if Company “A” hired a bunch of Head Hunter agencies to recruit people for them….that if you happen to apply at one of those head hunters and you are an employee of company “A”, that, that Head hunter can leagally call your employer to give them a heads up?!

I just heard that this happened with a company out there. Two fo their employees in fact. One guy ended up leaving and the other women stayed with a warning.

Is this legal…apparently this company had this written into their contract with the headhunter.