Is this use of the technology possible?
It depends on what you are trying to achieve. If you have two tagged items within an area covered by a reader antenna, you could have the reader issue an alert if one item were to leave the read field. Ordinary passive ultrahigh-frequency (UHF) antennas, however, do not give you a perfectly defined read field. So if a tagged item were to be moved to the edge of the read field, it would likely be interrogated sometimes and not at other times.
If the defined area is a small section of a shelf—say, 4 square inches—then a high-frequency (HF) reader can provide a well-defined read field. This would allow you to set up software to alert you if an item left that 4-by-4 space.
If the items are large and of high value, you could use an active ultra-wideband (UWB) system. UWB provides the greatest location accuracy of any RFID system, down to a few centimeters. So you could put a tag (which costs around $50) on each item and set up software to sound an alarm if the items moved more than, say, a foot apart.
—Mark Roberti, Founder and Editor, RFID Journal