A method and a device are disclosed for reliably detecting fluid leaks while minimizing false alarms. A leak sensor in proximity of a flat leakage surface, such as a floor area, is used to sense a permittivity of a media, such as water, coming in contact with the leak sensor. Leak is indicated if the sensed permittivity exceeds a permittivity threshold within a predefined time period. The predefined time period indicates a rate threshold that if exceeded indicates relatively fast accumulation of fluid in an air gap between the leak sensor and the leakage surface, precluding or reducing the possibility of false alarm due to gradual increase in environmental humidity or moisture. The air gap defines a fluid volume that is substantially filled before leak is detected.