e.g.
http://www.amazon.com/NETGEAR-VueZo...TF8&qid=1424445063&sr=8-1&keywords=vue+camera However, this camera is expensive and I've seen its image quality. It's not great. That is simply not the camera's fault; having a truly wireless, battery-only setup is a very tall order.
Another approach you could use would be to run something on a battery pack. The simplest approach that comes to mind is buying a large USB battery pack on amazon, e.g.
http://www.amazon.com/RAVPower-1500...d=1424445127&sr=8-5&keywords=usb+battery+pack and using a Dropcam, because Dropcam uses wifi for network, and it is powered only by USB.
I've not run the numbers on how long that pack would run the camera for, but I bet it's a good number of hours. Would have to be recharged daily, probably.
Even though the pinhole camera fenderman mentions is pretty much custom built for what you want (discreet camera), you could hide many cameras within boxes, in book spines, whatever, if you really want.
The camera he links has an SD card for local storage. That will record a few days worth of footage onto a single card, so now all you need to do is power it, either with a socket hidden somewhere or, I suppose if you absolutely had to, you could rig up a car battery or something. Motion detection works well and you can have the camera record only motion events, which would let the SD card hold many days worth of data. It will automatically loop over old data, like a dash camera for one's car. It also has higher image quality than the vue or the dropcam.
I think in practice you'll find your requirement of truly wire-free is not really so important.