Smart home tracks network devices by mac address
One surprisingly useful component of my home automation system is its ability to track every device that ever connects to the router at home by mac address.
Every few minutes my smart home scans the local subnet looking for connected devices. It does this by pinging each IP address in the local address range and whenever it finds a device it gets its mac address and compares it to a list of known mac addresses. Once it's found a device it pings it more frequently to check that it's still connected to the network.
Whenever it finds a new device (for example, a friend visiting with a cell phone or laptop that connect to Wifi) it can ask for that device to be identified (using the chat interface) and can track its comings and goings from then on. If you happen to enter the house with a smart phone with Wifi turned on you have just become an unwitting part of my experiment and your phone's mac address will be tracked from here on out.
Many consumer electronic devices now connect to the internet and each is thus tracked by the home automation system to determine how long it has been on for each day. This is used to calculate the instantaneous power consumption of the home along with the lights and other devices that the home automation can control or monitor.
Aside from estimating power consumption and tracking visitors to the house, one of the most useful aspects of this feature is that it's great for finding lost devices. Did your child lose their iPod touch at home or is it at a friend's house? A quick look at the log shows when it was last seen in the house confirming that it's still inside somewhere. Ultimately it might be able to cross-reference which rooms were occupied with when the device was last seen and thus give you an idea which couch or bedroom to go search (future feature), but for now, at least you know you aren't searching in vain. Ideally I'd have three access points with firmware that can track the signal strength of each device in the home so I can locate it more precisely but I haven't found a router offering that capability yet. My laptop can track and report how far away from the router it is but that's not as useful as being able to track individual devices from the router.
Another possible use for this is tracking screen time - how many hours did your kids spend watching TV or playing on the XBox when they were in fact meant to be working on their homework? etc. etc. Of course at some point this may all become a bit too Orwellian but as I've mentioned before, part of the experiment I've been conducting with my home automation is 'technical' but the larger portion is 'social' - which features are genuinely useful and which are just too much.