Two men in white overalls were seen entering a tent which has been erected on a spot of interest to police in Praia da Luz on the Algarve.
The area, which was covered by undergrowth until yesterday, has already been subject to fingertip searches by officers in Metropolitan Police uniforms.
Two white gazebo-style tents were put up and connected together to hide the scene from the TV crews and press watching from the other side of the police cordon nearby.
They placed it on top of a spot where a piece of corrugated iron was discovered beneath undergrowth yesterday.
The section of metal, which is understood to have covered up a void in the ground, was then taken away by officers.
Local forestry workers have been clearing large areas of undergrowth within the scrubland to aid the search. Another patch that was previously overgrown was earlier scanned with ground-penetrating radar equipment to probe for disturbed earth.
A man in plain clothes and without any police insignia wheeled the device along a section of ground.
The device uses radar pulses to take images of the subsurface of the ground to check for any anomalies.
It can be used to check for disturbances in a variety of substances, including rock, soil, ice and fresh water. It can also detect voids and cracks in buildings and under pavements.
The developments came during a third day of investigations by British police and their Portuguese counterparts as they search for clues as to what happened to Madeleine, who vanished while on holiday in the resort in May 2007, aged three.
The radar device was later brought out again in the same area as before.
It was moved along the ground by a woman dressed in black, accompanied by the same man as earlier.
A white tent was also brought over to the spot from another position as the device searched the ground nearby.
Five officers in forensics suits and face masks were later seen going into the tent.
A man in plain clothes and also wearing a mask then took a wheelbarrow inside and out again with a cover on top of it, disguising what was inside.
He repeated this for a second time before disappearing into another tent 100 yards away.
Banging noises and voices could be heard from inside the tent.
The Met's Detective Chief Inspector Andy Redwood, the senior officer investigating the case, approached at one stage and looked on from the slope above.
He declined to comment to reporters when he arrived at the scene this morning.
After around an hour of being inside, the forensics team came out and peeled off their overalls before walking away from the area.
The man with the wheelbarrow continued to go back and forth between the tents, transporting what appeared to be soil.