An object to be displaced in an oil or gas well includes a body that is movable through a conduit of an oil or gas well and an electrical circuit that is connected to the body. The electrical circuit can include an active or passive radio frequency identification tag, at least the passive type of which is responsive to an externally generated alternating current electromagnetic signal such that the electrical circuit transmits a responsive signal for reception outside the object as the object moves through the conduit. A detector apparatus for an oil or gas well generally comprises: a body movable through a conduit of an oil or gas well; an electrical circuit connected to the body, the electrical circuit for transmitting an electromagnetic signal for reception away from the body as the body moves through the conduit; and a receiver disposed relative to the conduit and the body to receive the electromagnetic signal from the electrical circuit. The apparatus can further comprise a transmitter disposed relative to the conduit and the body to transmit into the conduit an alternating current electromagnetic signal, wherein the electrical circuit transmits its electromagnetic signal in response to the alternating current electromagnetic signal from the transmitter. Related methods are also described.