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Something registered in the head of the tall Chinaman with a wispy beard and inscrutable stare when I introduced myself. It was impossible to tell what exactly, but I don't think it was positive.

Maybe it was my informal greeting, which consisted of a couple of friendly pats on his generous stomach followed up with "So, where's the Harley, big boy?" Or perhaps he was disappointed I hadn't joined him for breakfast. Of course, I might have been being overly sensitive. Whatever, suffice it to say that Ai Weiwei was not effusive when we first met last autumn.

Things got better after that awkward start. He likes Marcel Duchamp and so do I. That was enough. We spent a day together , during which time we talked about everything from marriage to moguls. I was looking forward to seeing him again today at the . But of course I won't, because he has vanished, having been thirty-eight days ago. There has been no word from him since.

And for a man who likes to tweet, blog and take photographs pretty much constantly, that is a bad sign. It makes his show at the all the more poignant. His brightly coloured vases look like eager children waiting for their dad to come home. The empty chairs and marble CCTV camera on the other side of the gallery suggest they could be in for a long wait. The empty coffin is chilling.

Nobody knows what he has done wrong beyond what the Chinese have called "economic crimes". Nobody knows where he is and when he might be released. Friends, supporters and fellow artists are becoming increasingly worried. All this at a time when many of the world's leading museums and galleries are strengthening their ties with the Chinese authorities; arranging exhibitions, sharing expertise. Will they now cut those ties, or at least loosen them?

Ai Weiwei is a significant man. He demands respect through the force of his personality and intellect. Like many powerful people he talks very quietly. Why should he speak up? If you want to hear him then lean forward and listen. He makes few concessions.

His entourage (not a big one, but an entourage all the same) tends to encircle him, partly to protect, partly to be close. But when he is being interviewed they melt away into the background. Except for a young American documentary filmmaker who has been Ai Weiwei's constant companion for many months, recording his life in minute detail. She is always nearby, even when he is being interviewed, there she is, three feet away documenting everything that is said.

Which in Ai Weiwei's case is often direct and honest. He told me that the famous series of photographs he took of himself dropping an ancient Chinese vase was intended as a joke, not an artwork. And the only reason they came to be considered a work of art was out of necessity. He desperately needed to find some work for a solo exhibition and he didn't have very much knocking about at the time. So, he fished out the photographs and offered them as something of a filler. Today the photographs are one of his most celebrated creations.

He told the story with a broad smile, which is not a face he pulls that often. It wasn't a smug or cynical smile, but one of faint bemusement: the smile of a creator who accepts that others are sometimes better at judging an artist's work. Very Duchampian.

His manner changes when he talks about China and politics. He gathers himself physically, his eyes narrow, his stare becomes more intense. He speaks even more quietly. And he refers to himself in the third person, suggesting his approach to political change is replicable - perhaps even a manifesto. He told me that one Ai Weiwei can't change things but if there were 100, maybe 1000s, then the government would have to listen.

Well, today there is no Ai Weiwei, he has disappeared - all is eerily quiet.