Apple has filed a patent for "generating notifications based on user behaviour" to determine whether or not the person in possession of the device at any given time is actually the person who owns it.
The system relies on learning the patterns and behaviour of the owner by compiling data on any sort of input and interaction. Whether that's locations that you frequent, how you walk, or even how you type, your device will be recording it to learn more about how you interact with your phone. Don't trust Apple with this data? It will be possible to turn parts of it off, if it ever becomes an eventuality.
Once the device realises that an unscrupulous individual has taken possession of your phone, the servers will lock it remotely until the real user verifies who they are, whether that's with a password or a fingerprint. Alternatively, the server will send a message to a trusted third party to let them know that something fishy is going on.
It'd be great if your phone or tablet might one day be able to realise if it's been stolen, but would you want to opt-in for that? Seems rather SkyNet-y to me. [Apple Insider]