A gas detection apparatus mountable to an unmanned aerial vehicle (UAV) comprises a transceiver module, a reflector module and an electronics module. The transceiver module comprises a laser emitter and a laser receiver; the laser emitter is tunable to emit a laser spectroscopy beam that can detect at least one target gas, and the laser receiver is configured to convert the laser spectroscopy beam into absorption spectroscopy measurement data. The reflector module comprises a reflective surface capable of reflecting the laser spectroscopy beam emitted by the laser emitter to the laser receiver. The transceiver and reflector modules are mountable on parts of the UAV such that the transceiver and reflector modules are spaced apart and the laser emitter and laser receiver have an unimpeded line of sight with the reflecting surface. The electronics module is communicative with the transceiver module and with a flight computer of the UAV, and comprises a gas detection program that determines a concentration of the target gas from the measurement data received from the transceiver module; when the determined concentration of the target gas meets or exceeds an alarm threshold, the program records the received measurement data and instructs the flight computer to execute a defined flight plan for the UAV.