An offender monitoring system has an electronic tag and a monitoring unit. The electronic tag has a tether to secure the electronic tag to an offender and a communication device to communicate with the monitoring unit. The monitoring unit has a communication device to communicate with the electronic tag. The system has a processor which analyses a communication between the electronic tag and the monitoring unit to determine a communication property, such as latency, variability of latency over a plurality of communications and signal strength. The processor determines, based on the measured communication property differing from a historical value, whether the communication between the electronic tag and the monitoring unit has been relayed via an intermediary device in an attempt by the offender to trick the monitoring unit.