His mother Manuela, 53, said: "He held on to me really tightly and he began to tremble. He said 'Mummy, take me home. I was locked away for two and a half years and I couldn't get any fresh air'.
"He was in a completely squalid condition when he came out of the flat. We both cried. He took my hand and wouldn't let go.
"I always said he was either dead or somewhere he could not get in touch with me.
"I had pictured to myself for two and a half years how it would be to see him again."
Marvin was 13 when he went missing after being taken into a nearby care home after the death of his father.
He said goodbye to his carers before leaving to meet friends on June 11, 2017 in the town centre.
He sent a final WhatsApp message at 11.37am and before his phone was permanently switched off.
Police had been prepared to give up. In July he was mentioned on on Aktenzeichen XY, the German equivalent of CrimeWatch, in an appeal for information.
His discovery seems to have been by accident. At 7am on Friday police visited the home of Lars H with a van and a dog trained to sniff out hidden computer hard drives - suspected of containing child abuse images.
They found binbags full of used nappies - and an overwhelming smell of urine. The smell was so severe investigators had to work with face masks on.
And when officers opened a cupboard door an uninjured Marvin was standing in the darkness.
"I'm so exhausted," his mother said shortly after meeting him while in psychiatric care.
"I was allowed to go into the room and spend ten minutes with him. He's exhausted, too."
Lars H and a 77-year-old man - reported to be his father - were arrested.
The older man was released but Lars H, an unemployed handyman, was kept in custody over the weekend. It is unclear whether he will be charged.
