A quick walk through Wanchai Computer Center or Golden Computer center up in Shum Shui Po will show you dozens of varieties of webcams and cameras. I'm assuming you are going to want something with some remote functionality (i.e. viewing via the Internet from your office, or over your iPhone or something). You might look around some of these stores, perhaps focusing on a wifi-enabled IP camera. This camera will exist as a stand alone device on your home network (do you have wifi in your house?) and you can set it up so you can access it via a special webpage to see what is going on in your house.

Or you looking for something more discrete, like a teddy bear camera? Not sure where to get those.

I would also urge you to start planning for nanny 2. If you hire someone there is a chance she may have to go back to the Philippines or wherever and get some paperwork sorted, which can take a couple of weeks. You might get one who is allowed to just go to Macau and come back, but sometimes they have to go home first and that takes time.

I also wouldn't be opposed to firing the helper as soon as possible (i.e. when you have a replacement ready). She's not going to get more serious or more committed to her job the closer and closer she gets to the end of her contract. I would expect more incidents like this.

Good luck.