As mentioned, it can be silent until receiving specific signal. It can be single frequency it can be multiple, but if it's specific string of bytes or specific

As mentioned, it can be silent until receiving specific signal. It can be single frequency it can be multiple, but if it's specific string of bytes or specific timing it's practically impossible to brute force when it's long enough, and when looking for the plane you can broadcast it with high intensity because it is only single use.