It's a good idea, I was musing something similar recently actually. So, you want to use a sensor type that a common smartphone can read, BT and Wifi seem to be

It's a good idea, I was musing something similar recently actually. So, you want to use a sensor type that a common smartphone can read, BT and Wifi seem to be the best choices. Power wouldn't be a major issue, they would be plugged in to a wall outlet, or possibly even powered from a PoE line. My mind is thinking about triangulation and the like. If the BT devices were networked, I believe you could have them transmitting an ID and a medium resolution time stamp. The phone would also have to be using network time and you could get a fairly accurate triangulation (again, I think, this part is far from my speciality). There is a lot of merit to this idea. You could roll this into something to do with digital signage and advertising as well. As stores in a mall come and go, they have to update directories. You could build a system to let a mall auto-update it's layout maps as tenants change.