Does it matter how small they are?
RFID transponders have metal antennas and would thus appear on an X-ray, regardless of their size. Keep in mind, though, that a transponder’s size relates to its read range—the smaller the antenna, the shorter the read range. So if someone were to create a very tiny tag, it would only be readable from within a few millimeters, which would make the tag useless for tracking anyone—though it would still show up on an X-ray.
—Mark Roberti, Founder and Editor, RFID Journal
Which Antenna Should I Use? »