Can such tags be thin?
RFID transponders designed to work on or near metal usually have a spacer cushion preventing a tag from being detuned. When you touch a metal hanger to the antenna of a transistor radio, the antenna is detuned and you hear a lot of noise instead of a clear broadcast. Similarly, an RFID transponder's antenna becomes detuned and cannot receive energy from a reader. The cushion prevents such detuning from taking place. Tags might have a specially designed antenna as well, to help improve read range.
At present, I am unaware of any thin tag that can lay flat on metal. However, that could change within a few years. Researchers at North Dakota State University's Center for Nanoscale Science and Engineering (CNSE) have developed an antennaless RFID tag that essentially transforms a metal object into a device's antenna (see The Object Is the Antenna). The chip and a small loop would lay flat on the metal. Such an innovation, however, is still in the research and development phase.
—Mark Roberti, Founder and Editor, RFID Journal