ALL the parking sensors should ALWAYS come on automatically immediately when you select REVERSE gear, I.e. both front and back will come on together. The sensors will stay on until the speed of the vehicle exceeds about 5 mph at which point they will switch off. So, for example, if you are parallel parking with a reverse movement as your first maneouvre your sensors will work both front and back as you shuffle into the space.
However if your FIRST manoeuvring movement is FORWARDS e.g driving forward into a parking space then you need to activate the sensors MANUALLY by pushing the button on the dash. The sensors should not come on automatically if you have not first engaged reverse gear. This is the way they have worked on my last 5 cars so seems to be the industry accepted procedure.
They should never come on automatically at the front otherwise they would activate every time you pulled up behind a car in queuing traffic.
That's good to know, exactly what I am used to as well.