Do active RFID tags exist that have the ability to read passive tags within their immediate vicinity, before transmitting to a reader?
It is possible, but for most applications, it probably would not be practical if such an active tag were to remain small—that is, no more than several inches in size, which is the size of a typical active RFID tag. That's because a tag's small size could not accommodate the battery power and antenna required to read passive ultrahigh-frequency (UHF) tags located more than 12 inches or so away.
For example, tag-size mobile RFID readers, such as the key-fob Bluetooth UHF RFID readers offered by CAEN RFID and Serialio.com, have small antennas and, therefore, offer only a short read range (10 to 90 centimeters [4 to 35 inches], depending on the reader).
The short answer, then, is: No, I am not aware of any active tags that can read UHF passive tags.
—Mark Roberti, Founder and Editor, RFID Journal