The 37-year-old woman was flying to Amsterdam in the last week of March 2019, less than six weeks after Leah went missing.
As she was waiting in the queue for passport control, her attention was caught by the couple directly in front of her.
"My first thought was ‘what a lovely young girl’ and my second was that the man she was with was being very controlling,” she said.
"The girl was asking him questions about what they would do when they arrived and where they would stay. She seemed worried. The man kept telling her: ’You’ll find out that when you get there.’ His tone was not nice.
"But what really made me suspicious was when the man said: ‘Do not say you are travelling with me. Say you are travelling alone’. He didn’t just say this once, he said it several times. I thought it was very strange.”
The businesswoman paid particular attention when it came to the couple’s turn to show their passports.
"The man went first and said he was travelling alone. Then it was the girl’s turn. I heard her say her name was Leah but I didn’t catch the last name,” she said.
“She also said she was travelling alone. But at the other side of the barrier rail I could see the man waiting for her. She went through and they walked off together.
"It just didn’t make sense to me. As a mum, I couldn’t help worrying about this girl called Leah.”
The witness was living in Holland at the time and knew nothing of the Leah Croucher case.
But six months later, she came to the UK to live in Milton Keynes and she saw the giant banners Leah’s family had put up on bridges.
"As soon as I saw the picture, I thought ‘that’s the girl I saw at the airport!’ I realised it was something I had to tell police about quickly.”
Following the instructions from police appeals, the woman made an online report on the Thames Valley Police dedicated Leah page. She gave brief details of what she had seen and expected police to contact her for more information.
"I heard nothing. No police officer called me. I thought it was odd as I’d even offered my passport details so they could easily check who was at the airport that day.”
Since then, the woman says she has made two more reports to police about her sighting. One was made online in 2020 and the other was a seven minute phone call to 101 in December 2021, of which she has a record on her phone.
But she says she has still not been contacted – and she remains convinced that it was Leah she saw that day.
"I am positive it was Leah. Her hair was an auburn colour and it was long, very thick and slightly wavy. She was wearing dark framed glasses.
"She had light make-up on and was dressed in black and a chestnut colour. She was wearing a skirt that came just above her knee and calf-length boots with block heels.
"I am 1.6m (5ft 2in) tall and I would put her at about my height.”
The woman described the man she saw allegedly accompanying the girl as ‘Asian, with gelled black hair and a trimmed beard. He was wearing a leather jacket that was hip length and it was black or dark brown’. He was in his late 20s or early 30s.
"He was not tall. And he had a south Asian accent but not a strong one,” said the witness.
Frustratingly, she cannot remember where the couple said they were travelling to.
"I remember each of them was asked but I can’t remember their reply. I’ve even thought about going under hypnosis or something so I can remember more.”
The possibility that Leah may have travelled abroad has not been a focus of the investigation – mainly because Leah left her passport, along with all her other possessions – at her Emerson Valley home after she disappeared.
But many people have speculated since that a fake passport could have been used had she left the country.
Leah’s name has never featured on the Interpol Yellow Notice list of missing people. In January this year, the Citizen asked the police why?
Head of the investigation DCI Andy Howard said at the time: “There is no evidence to suggest that she has left the UK."
Following contact from the Citizen, police say they will follow up the report of the airport sighting.
A spokesman for the Leah investigation team told us: “We have recently received a number of reports from members of the public relating to the Leah Croucher investigation. One of these reports relates to a possible sighting of Leah at Luton Airport in March 2019, however the circumstances reported to us are different to those in your request for comment.
“As with all information reported to the investigation it is assessed and allocated for follow up enquiries where necessary.
“We are currently in the process of following this report up, along with a number of other reports, and will be looking to speak to the caller shortly.
“There is no record of the caller contacting the police investigation prior to this recent report as suggested."
Leah Croucher was 19 when she vanished inexplicably while walking to work on the morning of February 19 2019.
Her bank account was not touched since and her phone left the network at 8.34am that morning.
If you know anything, please contact police on 101 or Crimestoppers anonymously on 0800 555 111. There is a £20,000 reward.