You can make a simple Knock Alarm using a Piezoelement. The Piezo element used in buzzers has a film of Piezoelectric material called Lead Zirconate Titanate. The crystals of this material generates around 1 volt when it is mechanically or thermally stressed. This principle is used to make this vibration sensor.
MQ 6 Gas sensor is used as LPG, Butane and Propane Sensor to generate alarm when there is leakage of these gases from the source. MQ6 is a highly sensitive gas sensor to petroleum based gases but less sensitive to Alcohol and Carbon di oxide. This simple sensor can be installed in the kitchen to give warning alarm if there is leakage of LPG.