I took a look at your user manual and looked up the LPG part number. The photo of your LPG monitor looks very similar to this
one. Flashing green indicates the the detector is working. They flash the LED to conserve energy. The fact that it's chirping and flashing green isn't addressed.
LPG detector only have a finite life. The sensor is heated to a high temperature in normal operation and the sensors typically only last 7 years. The later model LPG detectors even keep track of time to alert when the sensor are no longer reliable. I would replace your detector with a new one.