The groggy-looking blonde girl appeared alienated from the German family she was staying with at the resort on the Costa del Sol, according to a witness.
The potential sighting came three days after Madeleine McCann was abducted from Praia da Luz in Portugal, a few hours drive away.
British detectives investigating her disappearance have been told the German family paid extra after arriving at the campsite with one more child than expected on May 6, 2007, reports The Sun.
It is also believed that their vehicle had fake number plates to avoid detection.
The alarm was raised by holidaymaker Karen Sisson, 49, who was staying at the Cabopino campsite near Fuengirola.
Karen Sisson, from Gibraltar, told police at the time that the girl was wearing a pink T-shirt, shorts and a beige hat and seemed “out of it”.
She said that the girl seemed out of place with the German couple and their two curly-haired children.
Karen Sisson’s suspicions were further raised when she saw the girl wearing the same outfit the following day.
The mother-of-two left the next day but records show the German family stayed at the campsite for two weeks.
They checked in under the name Karsten Mayer, who gave 1970 as his year of birth.
Their car had a “BE” German number plate, which went out of official use 38 years ago – but the vehicle was a Mercedes Vito people carrier that was first manufactured in 1996.
A spokesman for the McCanns said yesterday: “This lady has done the right thing in reporting what she saw to the police.”
The development comes as Madeleine’s parents Kate and Gerry McCann on Sunday declared they will never give up their search for their missing daughter as they took part in a ten-kilometre charity run.
Just three days after the fifth anniversary of Maddie’s abduction, the couple took part at the busy Liverpool event for the first time.
Kate McCann, 44, wore a white vest with a picture of Madeleine on the front and the words “Never give up” on the back.
It was the first year they felt able to take part in the event.
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