I'm divided on the "spy camera" issue here. I would consider installing one. As far as the legality of this, I'd have to research HK laws more but this city is so full of surveillance that it stands to reason that having a camera on your own property in your home that records things, whether or not your helper knows, probably would be okay. How many of us are recorded without our consent or knowledge when we are on private property in this city. We'd all be shocked if we knew.
My mom has a great thing that she says:
"Locks keep honest people honest."
So, I think it's a good comparison. There are some things in our home (cash, jewelry, equipment) we keep in a small safe. Do we keep them there because we "can't trust our helper"? No. We keep them there because "locks keep honest people honest." We trust that our helper is overall honest and wouldn't steal from us. However, we all realize that human nature is human nature. Maybe she's got personal financial problems that would lead her to steal--or maybe it was a desperate situation. We don't think she's going to steal from us but we still keep our important belongings out of sight and locked up.
Same thing with our child. He is far more valuable than any material possession we own. He is priceless. Do we think our helper is going to behave inappropriately toward him? Overall, we trust that she won't. But, as the original poster said, the helper is new. Trust is earned over time in some cases. So, it doesn't sound completely inappropriate to check up on her performance if you're not around. At least at first.
Anyway, no, having a security camera probably won't give a person 100% peace but they can come in handy, I think.
Having said this, we have not yet installed a security camera. We'll see how we deal with this question when our new helper arrives.