it might be a microwave detector, which can sometimes be activated through walls (more likely through the wooden door). They may be specified for local authority contacts because the sensor can be concealed making them less vulnerable to vandalism than a PIR
http://www.elkay.co.uk/product.asp?id=52&catid=1
Or there might be a loose connection that is affected by vibration.
http://www.elkay.co.uk/product.asp?id=52&catid=1
Or there might be a loose connection that is affected by vibration.