From the article RFID in Retail Stores: Suggestions for Eliminating Smart Shelves:

In retail stores and other environments, the inability to rapidly locate items is a common problem. Retailers may appear to be out of stock of a product, when in fact the product may be available in the back of the store or may have been placed on the wrong shelf. RFID (radiofrequency identification) technology has been proposed as a means to improve the ability to track inventory and to locate objects. In particular, the use of RFID-tagged objects coupled with smart shelves that include RFID readers has been proposed as a means of efficiently tracking the presence of products in a retail environment. However, the smart shelves that have been demonstrated in public trials have employed numerous expensive RFID readers adapted to read sections of a single shelf, and have required the use of expensive and bulky coaxial cable for each of the readers… In this paper, we propose several alternative systems that may be considered.

The alternatives include techniques for extending RFID signal range, use of active RFID tags with an inherently greater range, a limited number of moving RFID readers that scan RFID tags, and the like.