Missing Leah Croucher was almost certainly in a secret relationship with an older man before she disappeared, the Citizen can reveal.
And her family had warned her off the man, who was engaged or possibly even married at the time.
For legal reasons, we cannot reveal the man's identity but police have spoken to him and have no suspicions about him. It is understood he has an alibi for the time that Leah vanished.
Today Leah's mum Claire broke her silence to talk about the relationship with the man, who we will call X.
"It started in the summer of last year. She was talking about this man all the time. It was X this and X that ... it was obvious she had a soft spot for him.
"She'd go and see him in the evenings, paying £13 each way for a taxi. It just wasn't like Leah to do things like that - she'd always been such a home-loving person and didn't like going out much.
"Then we found out he was engaged and it sounded like it was an arranged marriage. I remember saying to Leah: 'Don't you go falling for him, he will never be yours.' I was under the impression that she'd stopped seeing him then."
Now, in hindsight, Claire and her husband John release Leah could have been deceiving them.
"Her behaviour changed, ever so slightly. I didn't really take much notice at the time but now, having had eight and a half months to think about it, I realise she had become a bit more moody, which was totally out of character," said Claire.
From September 2018 onwards, the normally placid 19-year-old became "a little bit snappy", slightly more argumentative and would go up to her room and sulk. But her parents put it down to normal teenage behaviour.
"Leah had never been like many other teenagers. She was happy to be at home reading her Twilight books about vampires. She didn't like nightclubs and I had to force her to go to the pub after her 18th birthday. She finally went but came home saying it was too expensive and not much fun!" said Claire.
On Saturday February 3, twelve days before Leah disappeared, she announced she was going to book a hotel room and have a girlie night with two female friends, drinking alcohol and gossiping.
"To be honest, I didn't think anything of it. I was pleased she was getting out and it seemed quite a normal thing for young people to do. I know her brother has done the same when he goes for a night out," said Claire.
Leah's dad dropped her at the Jury's Inn at Central Milton Keynes and waved goodbye as she walked off.
It was only much later, when her bank statement arrived some time after Leah's disappearance, that the Crouchers realised their daughter had lied to them.
"Her statement showed she had booked a room at the cheaper hotel nearby - the Travelodge," said Claire. "We checked with her friends, of course, and they said they hadn't been with her that night.
"We believe she was with X, but did not want to tell us."
Frustratingly, CCTV of the Travelodge had been wiped out by the time the lie was discovered, so there is no record of the assignation.
Claire and John are asking anybody who was in that area on February 3 between 6.30pm and 8.30pm to rack their brains to see if they remember spotting Leah. It is possible she and her companion went for a drink or food beforehand in one of the nearby pubs or restaurants.
"She was wearing her black coat and, we think, black jeans. She took her black rucksack - the same one she had with her when she disappeared - with her overnight stuff in," said Claire.
Looking back, she realises there have been other little clues that Leah was having an affair and had perhaps fallen head over heels in love for the first time.
Then there was the mysterious outing on Valentine's night, the night before Leah vanished. She left home between 6pm and 7.15pm, telling her mum she was seeing a friend. But checks showed she never saw that friend and police, despite numerous appeals, have never established where she went or why she was with.
"You'd think there would be evidence on her phone records of a secret relationship, but there is nothing," said Claire, "But then we know she always used Snapchat, and chats are wiped out on that. Or, of course, the man could have supplied her with another phone, a secret phone."
Claire admits she wavers desperately between two theories. The first is that Leah ran away deliberately with someone she knew and has either chosen not to return or is being held against her will. The second, too awful for any mother to contemplate, is that she was snatched off the street and is no longer alive.
"I try not to think of the second one, but as time goes on it's there ... I get bad days where it won't go away and I cry and cry. I desperately want her to be alive but we are going through absolute hell.”
Claire tries to comfort herself with the fact that Leah was an expert at self-defence, and has studied at her dad's martial arts club since she was a young child.
"How would anyone manage to take her against her will without anybody else seeing anything?" she said. "I've walked the route she took to work and it's busy - you are visible to other people and passing cars all the way, particularly at that time in the morning when everyone is going to work or school.
"So then I try to convince myself of the first theory - that she has run away - and I think why would my daughter choose to cause so much hurt to us, the people she loves. It's just not what Leah would do and it doesn't make any sense at all."
But then Claire remembers Leah as a little girl, talking about her dreams for the future.
"She's always been crazy about vampire books and she used to say that all she wanted was to meet a 'bad boy' vampire and be with him," she said.
Claire says her family discuss theories endlessly, but end up "going round in circles" with not a single clue as to what has happened to Leah.
Her one consolation is that Leah knows her dad's mobile number off by heart because it was very similar to her own.
"We live in hope that one day it will ring and it will be her," she said. "No matter what she has done or who she is with, we will never stop loving her. There is nothing that we can't work out, and we want her to know that."
If anything here has jogged your memory, or if you remember seeing Leah with a man, please contact police on 101 quoting reference 43190049929. If you do not want to give your name, call Crimestoppers anonymously on 0800 555 111.
There is a £5,000 reward for information leading to Leah's return.