Sorry to hear about the scratch on the rear bumper.
Is it a big scratch or smaller one?Is it very well visible? If the gates are in the sensor limit it should identify the object. In my Honda City it detects gate. Please check the video in my thread.
I also made the same mistake by only looking at the screen. Some times we have to watch both side mirrors and screen. Use the screen only for identifying obstacles on the back side of the car which will be invisible through side mirrors.
The parking sensors have its limitations since when it goes beyond the limit, it wont detect any obstacle( I think this happened in your case). and the screen blinks when it starts to display distance(in my car reverse system, the screen will be blank when it starts counting the distance. this happens just for a sec or less than a sec) so I will move it slowly until it gives final information ( when the screen is back). you will get used to it after using for some time.
So my advice will be don't drive with 100% confident that you will never hit an obstacle while reversing by trusting rev camera and sensors.Sensors wont be able to catch smaller walls, trees, etc. ( as I mentioned before about the limit).
You can trust it 100% if the back side is of a flat things like fence, building etc..). Even after the sensor gives stop stop signal there will be a minimum clearance of 15cm(apprx.)
And be careful if the backside is sloppy. The sensors will not detect the slopes. in that case you may scratch the bottom side of bumpers.
the cam with sensors are helping much better than those with only sensors. it gives only pic pic pic sound which drives me mad.
And make sure that the sensors are working fine by checking with a flat, height obstacles like side wall, otherwise we have no chance of complaining Honda people.