What about Nannies?

So what about your house? Should you have a hidden camera in your home?
There is a great deal of controversy over this subject. Is it an invasion of privacy, is it legal, is audio allowed?
For some it may be different than others. If you are a new mother leaving your newborn with a nanny for the first time, I say more power to you! As much as I would miss that baby, it would make my day so much easier to be able to "see" him/her and make sure he/she's happy throughout the day. And maybe you do need to check in on that new nanny! So that nanny has glowing references and seems great with the kids while you're there. What about when you're away? Does she talk on the phone constantly or watch TV while neglecting your child? These are not minor things, in my opinion and should be addressed. What if there are questionable people stopping by your house to see her? What do they do in front of your baby, toddler, preschooler? Of course, you should use your own good judgement on this one, but can you really afford to have someone watching your child that doesn't have their best interests at heart?
More on the legal side....