Thats the most obvious one. What if someone wants to break into your house? pop a tile on the undersid of your car, and they know just how far away you are.
Dont think for a minute that this technology wont be mass marketed in some form other than a kinda obvious looking white tile. You could drop that into something that looks like a rock, or any other shape you want. Then, it will get smaller. Put them in all credit cards, insurance persuades you to have one in or on your car through a discount, Car companies installing them in each car they build, etc etc etc
Then think of the dangers of the database that holds the coding for all of the tiles being hacked into, or worse yet, sold off.
Bad stuff.