Vaillant, pump overrun?

We have an ecoTec pro 28, 3 years old. When cold water taps are turned off, when the central heating is off, status code S 17 appears and the boiler appears to power up for 30 seconds.
It has not always done that and when the tap is turned off slowly it doesn't happen.
Vaillant tech support say it could be a dead leg in the house pipe system but I can't find one.
Please can you help with any clue?