Puppy escape

I am not quite sure how it happened or when it happened but the puppies have discovered a way to get out of the garden.
On Saturday, the first my youngest knew about it was when he heard distant barks and Tess and Hardy were discovered legging it up the lane by the nursing home. I accepted that maybe a gate was left open as the puppies do not jump fences and none of the ducks were out.
On Monday, I did the duck houses and suddenly the garden was silent. No Tess or Hardy. I called and called. Th...