She also criticised how Portuguese police handled the incident.
She said: "[Kate] was pacing up and down. The worst possible thing had just happened to her.
"She was crying, but almost in a catatonic state, and Gerry was very distressed.
"That's the one thing I really remember from him, looking under the cars. I can't forget that."
The woman also claimed police missed out "whole chunks" of information during their investigation and took some 90 minutes to arrive at the scene.
She said there was "no way" the McCann's had any involvement in their daughter's disappearance.
Police investigating the disappearance of Madeleine were last month given an £85,000 (NZ$152,000) "last throw of the dice" after identifying a new "person of interest".
The hunt had been due to end within weeks but extra cash was found to explore a lead in the investigation, which is nearly a decade old.
An insider said police had found a person who was close by when the three-year-old went missing while on holiday in the Algarve resort of Praia da Luz in 2007.
The source said: "There is just one person who detectives want to speak to, who was near the area where Madeleine disappeared almost ten years ago. An international search has been under way to find them.:
The Portuguese press reported that 'police are understood to have clues pointing to the existence of a new suspect', but it was not said whether it was a man or a woman.
A report in newspaper Correio da Manha read: "This new suspect is believed to be Portuguese and was apparently seen near the Ocean Club, where Kate and Gerry McCann were staying with their children when Madeleine disappeared.
"British police are understood to be preparing to question this new suspect."
Kate and Gerry McCann were said to be 'very grateful' for the continued funding amid fears the £11.1million (NZ$19.8m) investigation into Maddie's disappearance, which is codenamed Operation Grange, was about to be shelved.
A family spokesman said: "Kate and Gerry remain incredibly grateful to the Home Office and the Met Police for the continued work into the search for their daughter."
A close friend of former GP Kate, 49, and heart doctor Gerry, 48, said: "They have never given up hope of finding out what happened to Madeleine, and believe she could be alive.
"They appreciate the Met Police's help and know they are doing everything possible for a resolution."