Home Automation - Is anyone home?

I have been interested in the idea of home automation for some time, I think most of us like the idea of things turning on or off as needed so that everything is ready when we want.
The closest I have gotten to actually doing something is the recent lighting project on my bed, the light now turns itself on meaning I don't have to!

The idea of a light turning on automatically may seem a little silly but it goes towards making the environment we live or work in nicer and more inviting. I had hoped to improve this system with an activity sensor which detected if anyone was around but sadly this didn't work.

The reason I am writing this post is because tonight I had an idea for a project which would tie into this.
One problem I have been looking at is knowing if I am at home or not, obviously I know but it would be nice if that data was available to other systems such as lighting or heating.
A switch or RFID tag could be used by the door to check in or out but this requires a physical action (which I will almost certainly forget to do) so I ruled it out.
Another possibility is to carry a small transmitter which paired with another device when I was in range, this is possible but does require the maintenance of the transmitter battery. The Nike+ device does allow this kind of hack and it has a very long life battery so that is probably a good choice for this type of detection.

The other option that I have been playing with using my phone and the bluetooth chip.
I enjoy the hardware side of projects like this so I could go down the app route which uses gps data or iBeacon style location but both of these leave you with the information on the phone, I want it at the other end.
This evening I have been trying to use an HC-05 bluetooth module to either scan its environment and detect my mobile device or alternatively have it auto pair when in range and then check this status. The problem I have is that the bluetooth module only seems to support serial links which the phone doesn't, its possible I could make it work but I don't have much time to investigate this further.

What I am going to do is leave this post as a reminder for myself; when I am in China I am going to try and pick up some bluetooth modules, including a low energy device which I believe uses a different system. My aim is to then come up with a simple piece of hardware that knows if I am at home and can then pass this data onto other systems such as the bed lighting, heating, or even a web service.