The friend broke his silence on the alleged incident after witnessing this week’s police searches.
Dozens of agents descended on a housing estate that Brueckner used to rent in Hanover on Tuesday.
They searched the site using sniffer dogs, penetrating radars and drones.
They apparently found a cellar under a stone slab and took away the concrete foundations of his old dumpster hut.
Brueckner is held in a German prison on suspicion of kidnapping three-year-old Madeleine from a holiday apartment in Praia da Luz, Portugal, in May 2007.
He was then living in the region. His lawyer called the searches “sheer desperation”.
His friend said: “We never thought his crimes involved children – he was a kleptomaniac. We just thought he stole everything in sight.
“He told me what he had done when he lived in Portugal. He said he survived by breaking into hotel rooms, stealing cameras and laptops.
“But the strangest thing he ever told me was to break into a room where four British teenagers were sleeping on the couch and the floor.
“He was naked and left his clothes outside and he was masturbating. When one woke up and tried to wake the others, he panicked and ran away.
“He said he had run naked through town. He told it like a funny story.
The pal said he wasn’t sure for sure if this had happened in Praia da Luz – but he believes it was in the same area.
He recounted how he befriended Brueckner at a German-owned kiosk in Braunschweig, northern Germany, in 2013.
“We were drinking in a back room. He spoke about his crimes and was quite proud of it.
The friend said Brueckner claimed he stole £ 100,000 from a house where the large deposit for the sale of a property was hidden.
He claims that Brueckner bragged about using the money to buy a house in Bavaria, where he lived as a child, and the kiosk. “It was all the thefts, burglaries and drugs he was talking about. He never mentioned the kids, ”the boyfriend said.
“He even claimed that the palm trees in his garden had been stolen. There was not a day when he did not steal something.
The friend said the German claimed he also used part of his £ 100,000 to buy a disused factory in East Germany at an auction.
Earlier this year, police reportedly searched the scene and found eight USB drives in a bag, some containing images of child abuse.
Brueckner ended up living in the Braunschweig Garden Housing Estate between 2013 and 2016.
Sabine Sellag, the owner of her old hut and land, fears she is sleeping over the body of another missing child known as the “German Maddie”.
Brueckner was questioned but was never charged after Inga Gehricke, five, went missing in 2015 near her home.
Residents said he enjoyed the open floors in the cabin shortly after arriving to gain access to an underground cellar.
New main suspect of Madeleine McCann
He was one of about six people living on one of the 164 plots, which is prohibited.
“A few homeless people live there, but they get away with it because it helps safety,” the friend said.
“He was nice to people. I think he was alone at the bottom.
But after three years, he was on the move again after police started questioning him about Inga’s disappearance and access to pornography on the web.
“I think he returned to Portugal, I never saw him again,” said the friend. “He sold everything he had. He bought a car to go with.
Brueckner is now making time for drug offenses in a Kiel prison and due out in January – but he has asked for an early release date.
He has yet to be convicted of raping a 72-year-old woman in Portugal in 2005.
He has previously denied any involvement in the McCann case.
His lawyer Friedrich Fulscher said he did not think he was charged.
“As long as my client does not know what he is accused of and on what basis, there is no reason to believe otherwise,” he said.