Volkswagen Passat rear parking sensors giving steady beep then cutting off

first thing check they are clean.as dirt can look like a solid object.
then they may have to be distance calibrated the better ones you can set the distance the solid beep comes in
otherwise remove side panel and check connections
its not unknown for the control unit to fail so dealer visit or local alarm/accessory company

To add to " mrsuzuki's " answer ,it could be water ingress . Check and clean the sensor contacts . The newer cars had a different designed sensor.