Airborne ultrasound detection can be used.
The PowerBeam 300 is a parabolic accessory originally designed to test high voltage insulator and transformers from a safe distance with a high degree of accuracy. The PowerBeam 300 can accurately pinpoint sources of ultrasound within 1 degree off center from up to 300 feet (91.44 meters) away.
The UL101 Leak Detector quickly locates the ultrasound produced by compressed gas and vacuum leaks. Simply aim the leak detector in the direction of the piping and adjust the sensitivity to pinpoint the location of any leak with turbulent flow. The UL101 has the best signal-to-noise ratio and highest sensitivity in the market. Leaks sound like rushing air in the operator's head set, and the gain can be minimized to eliminate all or most ultrasonic "white noise" for an operator-friendly user experience.