Not exactly disappearing, but my GM faked documents to make up a child. We didn't find out about until we were cleaning out her house.
The made-up kid was supposedly the son of my aunt, (her daughter) and my ex-uncle, who my GM was determined to get back together. He had moved overseas. This was in the 1990s. He didn't know about my birth, and it gave Nan an opportunity to make up their child.
Nan somehow got a copy of my baptism certificate, (from a church), used that to get a copy of my birth certificate (in person, I don't know how she talked her way into that), then used a colour photocopier and created new certificates for the boy she made up. Along with my baby photos, she sent that to her ex-son-in-law.
He contacted my aunt, furious that she hadn't told him about their 'child', my aunt denied it as she knew nothing about it. She asked her mother if she had told her ex they had a baby and Nan denied it. So my aunt thought the ex had gone mental, and didn't tell him when she moved / changed her phone number.
But he and Nan were corresponding - she told him that her daughter had lied to him, the boy existed. I don't know if he believed that, but he wrote back when she sent updates. When I went to preschool and primary school she picked a boy I was friends with and photographed him at events. She also used my school reports, which she requested photocopies of from my mother, and faked them to send to him. Luckily he was a disinterested father and never came looking for his son.
We didn't find out about this until she died. She kept a file with all the fake docs, including the forgeries in progress. I was 9 at the time, and I don't know what was said by my aunt - if anything - to her ex afterwards. But my mum has told me the story because it was my ID that was used, and I remember the drama.