there is no use to 暗示, no matter they understand or not, they will just ignore it. They always think we as their boss are dum person. I just told my maid that I definitely am cleverer than she is and I know how she behave no matter how good she seems to be in front of me. For your maid, I'll advise you block your home phone when you are out. You can also track the incoming number and return a call to the boss of the other maid. That means your maid's talking to her friend will also risk her friends job.