The Apollo Plate detectors where one of the first flush mounted smoke sensors, but were disscontinued 3 years ago, due to not meeting regs....Quite often architects request no surface smoke detectors on ceiling and voids, As this requires service hatches and is unsightly. So air sampling is used to detect smoke detection, small flush sampling plates measuring an inch are place through the ceiling, and are connected to the sample pipe via a small tube. System has a faster response then optical smokes, but is expensive....have only once been ask to install in a domestic dwelling, but customer was very happy with result.