The British girl went missing from a holiday apartment on the Algarve in 2007, shortly before her fourth birthday.
Detectives have not disclosed why officers travelled to Barcelona to meet with Spanish police between November 23 and 25.
However, The Evening Standard speculates that Scotland Yard might be looking into reports that the three-year-old was abducted and smuggled over the border from Portugal to Spain.
The review, named Operation Grange, has a total of 30 people working on it at present. It was ordered by David Cameron and home secretary Theresa May last year.
Officers have also recently travelled to Portugal three times in connection with Operation Grange.
A Scotland Yard spokesman said of the visits: “There has been very good co-operation with Portuguese authorities and liaison will continue.
“We are not prepared to discuss specific details of these visits nor speculate about any future deployments to Portugal or elsewhere.”
The official enquiry was shelved in 2008, after which no police force had been actively looking for Madeleine.
