On March 13th a man, probably in his 60s, collapsed while jogging through Volkspark Wilmersdorf in the west of the capital city. As he fell, he knocked his head off a stone and fell unconscious. Four months later, he still hasn't woken up.

By this stage worried family members, friends or neighbours would have normally made contact with authorities. Failing that, it is usually a fairly straightforward matter for police to track down his home and loved ones.

Not this time, though. The man is still lying in the intensive care station of the Charité hospital with no name next to his bed.

“We have never had such a case in Berlin,” a police spokeswoman said. “This is a completely new situation for our missing persons department. There is absolutely nothing to go on here.”

Investigators have tried everything in their attempt to figure out who on earth the man is.

Twice they have published photos of him, the first time without his prosthetic teeth, the second time with them. They have also published a photo of his key set, the only thing other than a couple of euros that he had in his pocket.

The keys have proved just as mysterious, though. The are produced by a large manufacturer but have no security code, a rarity which makes it impossible for the police to identify where the man lived.

Police say that the man was too well groomed to have been homeless. He was clean shaved, had healthy skin and had a trained, fit body.

It is possible that the man lived an extremely isolated life. But when police are confronted by such cases there is normally some tip off that puts them on the right path. An overflowing mailbox is reported by a neighbour, a doctor or dentist recognizes their patient’s face from the missing person’s report.

Another theory is that people know who the man is but are remaining silent.

In Germany at least, the man appears to have no skeletons in his closet. His fingerprint wasn’t found in police files, meaning he has never been suspected of a crime. An attempt to identity him via his DNA also hasn't produces results.

Another possibility is that the man was a tourist only staying temporarily in the city. But if that was the case, a hotel somewhere would have reported that a bill had been left unpaid and that a guest had left luggage in a room.

As long as no family turn up, a court appointed carer will make decisions related to the man’s well being. And, as it can not been established whether the man has health insurance, the hospital itself is carrying the hefty costs for his care.

At this stage it is also not clear how badly damaged his brain was during the fall. If he ever does wake up, it is possible that the man himself will no longer know who he is.