That is untrue. Passive high-frequency (HF) and ultrahigh-frequency (UHF) tags can be read through a blanket or a sheet.
Generally speaking, there are four things that prevent passive tags from being read. The first is distance. If a transponder is too far from a reader, then insufficient energy reaches the tag to enable it to respond—or, at least, to respond with enough strength for its signal to be received by the interrogator.
The second is water. This is primarily true of UHF tags, because liquids absorb RF energy at this frequency. Since the human body is composed mainly of water, if a person is lying on a UHF transponder, it will not receive energy from the reader, and will thus fail to respond.
The next is metal. Tags can not be read through metal, because the RF energy from the reader bounces off the metal and never reaches the tag. Metal can also detune an antenna if a tag is placed on metal. Even if a tag is facing a reader with metal behind it, it will not function properly since the antenna will be detuned. This is similar to touching a portable radio antenna with a wire hanger—you would immediately get static, because the antenna would no longer be tuned to the proper frequency to pick up a station.
The last issue has to do with material density. Even RF-friendly materials, such as paper or clothing, block some RF energy. The thicker the material, the more radio waves are blocked, so while a tag can be read through a single linen sheet, it might not be read through a thick stack of folded sheets.
—Mark Roberti, Editor, RFID Journal