It was only when someone in hospital had the bright idea of leaving him with a piece of paper and pencils that the first intriguing clue about the stranger's past emerged. He drew a detailed sketch of a grand piano. Excited, hospital staff showed him into a room with a piano and he began to skilfully perform meandering, melancholy airs. Several weeks later he has still not spoken a word, expressing himself only through his music.

Some who have heard the "piano man", as he has been nicknamed, believe he may be a professional musician. One theory is that he has suffered a trauma which has caused amnesia, one of the methods the mind uses to retreat from a shock. Personal memories can be lost while the ability to communicate - or, for instance, play the piano - is not.

The man's carers have become so desperate to find out who he is and what has happened to him that they have allowed his photograph to be taken in the hope that someone will solve the mystery.

The "piano man" was found on the Isle of Sheppey, Kent, last month. He wore a black jacket, smart trousers and a tie, all dripping wet. Police officers tried to find out who he was and if he had fallen into the sea, been pushed or even swum ashore from a boat - but the man remained silent. They dried him off as best they could and took him to accident and emergency at the Medway Maritime hospital in Gillingham.

Doctors examined the man, who appeared to be in his 20s or 30s, and found nothing wrong with him, but still he failed to respond to questions. He was difficult to assess as he appeared terrified of any new face, sometimes rolling himself into a ball and edging into a corner.

After hours of trying to elicit any scrap of detail about his life, someone had the idea of leaving him with a drawing pad and pencils. When they returned an hour later they found he had produced an excellent and detailed sketch of a grand piano. Realising that music might be the key to unlock the mystery, he was taken to the hospital's chapel, which contains a piano. The man sat down at the instrument and began to play. The doctors were amazed at the transformation. For the first time since he had been found on Sheppey he appeared calm and relaxed. He was also a good player - some say exceptional.

In the following weeks the "piano man" returned regularly to the chapel. He played sections from Swan Lake by Tchaikovsky but most often seemed to prefer to perform what appear to be his own compositions, which have been compared to the work of the Italian composer Ludovico Einaudi. Some hospital staff are convinced he is a professional musician and may even have been performing not long before he was found - hence his smart black clothes.

Canon Alan Amos, the hospital chaplain, said: "He likes to play what I would call mood music - quite circular in nature without defined beginnings or endings." Mr Amos suggested he was using music as an anaesthetic. "Playing the piano seems to be the only way he can control his nerves and his tension and relax. When he is playing he blanks everything else out. He pays attention to nothing but the music."

If allowed to he would play the piano for three or four hours at a stretch and at times has had to be physically removed from it because he refused to stop. When he is away from the piano he almost always carried a plastic folder with sheet music inside. Mr Amos said he did not believe the man was a professional musician, but someone who played well for his own pleasure. He suggested that he might have been wearing dark clothes on the day he was found because he had been to a funeral. He said: "It's a very sad case. Clearly there must have been some sort of trauma and it is important to find out what it was."

The "piano man" was eventually transferred to a psychiatric unit in Dartford, where he was given access to a piano. Manager Ramanah Venkiah said: "He has been playing the piano to a very high quality and staff say it is a real pleasure to hear it. But we don't know what his position is because he is not cooperating at all."

Research has suggested that exposure to familiar music can help people suffering post-traumatic amnesia. Some therapists offer music to help such patients recover lost memories and face the traumatic event which led to their state. Meanwhile social workers have issued a missing persons' bulletin on him. Until he is identified he will no doubt continue to play his sad but soothing music to the pleasure of those caring for him and his fellow patients.

· Anyone who has information that might help to identify the "piano man" should email steven.morris@theguardian.com