The development came as British officers, accompanied by their Portuguese counterparts, spent a second day investigating the hilly area in the resort of Praia da Luz on the Algarve.
Two sniffer dogs accompanied by British police spent around five minutes on a flat area of the scrubland, at the bottom of a slope.
They were then taken away out of sight, while around 10 officers stood around in discussion.
Specialist teams are thought to be using ground-penetrating radar equipment to probe the ground, looking for disturbed earth.
The large section of land is being guarded day and night by armed local police with dogs, and a large yellow-and-white cordon was put in place ahead of the search for clues about Madeleine, who disappeared in May 2007 aged three.
A number of small tents have been erected inside the cordon, thought to be where officers are concentrating their efforts.
The land, a few minutes' walk from the Ocean Club resort apartment where Madeleine was staying with her family, has been searched before.
The two spaniels were then brought out again and spent a few minutes in the long grass as their handlers looked on.
Around six Portuguese police vans could be seen on top of a large mound in the middle of the scrubland, which overlooks the sea.
The cordon was lowered down for two hired vans to drive in and they then reversed along a track before stopping near two blue tents.
TV crews and journalists from around the world were gathered at the cordon as the search - seen as one of the biggest developments in the investigation in recent years - takes place.