1. A radio frequency communication device, comprising: (a) a receiver configured to receive radio frequency transmissions; (b) a transmitter configured to make radio frequency transmissions; (c) an interface for receiving one or more signals from a sensor; and (d) electronic components arranged and configured such that, (i) the radio frequency communication device operates in at least two states comprising, (A) a first state, in which the radio frequency communication device responds to a radio frequency transmission that is received by the receiver, and that includes data representative of an inquiry as to the presence of radio frequency communication devices within an area, by making a radio frequency transmission with the transmitter that includes an identification of the radio frequency communication device, and (B) a second state, in which the radio frequency communication device does not respond to a radio frequency transmission that is received by the receiver, and that includes data representative of an inquiry as to the presence of radio frequency communication devices, by making a radio frequency transmission with the transmitter that includes an identification of the radio frequency communication device, and (ii) the radio frequency communication device enters the second state from the first state upon, (A) receiving by the receiver a radio frequency transmission that includes data representative of an inquiry as to the presence of radio frequency communication devices, and (B) making a radio frequency transmission with the transmitter that includes an identification of the radio frequency communication device, and (iii) the radio frequency communication device enters the first state from the second state upon, (A) expiration of a predetermined period of time as indicated by a timer of the radio frequency communication device.