A few years back where I worked they were trying to set up a paperless property tracking system. They considered several systems. One was a proximity sensor like you described; another would have an electronic tag attached to each item and sensors all over the ceilings of all rooms to track their movement. Then there was a simple bar code system. The proximity system was rejected because of cost and the fact that the receiver had to be very close. They were thinking of being able to just move the item through a door and it would sense it going through with no effort for anyone to scan it, but it wouldn’t work that way. The one with the sensors in the ceiling was popular because it could locate every item real time to with in a few feet. However each tag required a battery which would need replacing every 5 years or so, and with hundreds of thousands of items to track it would be just too expensive over time. The initial cost was higher by a large amount. Also the tags were bulky and could never fit on very small items. So they settled for a simple bar code system. Kind of a disappointment but for the time it was the wisest choice.