This year marks the 50th anniversary of the death of legendary American crime scribe Raymond Chandler, whose seven completed novels, including The Big Sleep, Farewell My Lovely and The Long Goodbye profoundly changed crime fiction and crime movies. The success of his novels - The Big Sleep was first to be published in 1939 - led Chandler to try his hand at screenwriting, notably with his debut screenplay, adapting James M Cain's sultry pulp thriller Double Indemnity with Billy Wilder, who also directed. Their collaboration was fertile and productive but also fractious. Chandler learned a lot from Wilder and Wilder managed to draw the very best out of Chandler. But they never worked together again and neither ever spoke fondly of the experience.

Now, however, more than 60 years after its release, a French cinema historian and two US crime-writers almost simultaneously happened on the same bizarre discovery - that Raymond Chandler, uncredited and previously unnoticed, has a tiny cameo in Double Indemnity. On 14 January, the American mystery writer Mark Coggins, tipped off by another writer, John Billheimer, posted the news on his website, Riordan's desk (tinyurl.com/raymondchandler), while the French journalist Olivier Eyquem, wrote about on his blog (tinyurl.com/chandlerfrench) on March 30.

Cameos, more often than not, are the preserve of famous actors or are in-jokes by the film directors themselves (most famous of the cameo-making directors is Alfred Hitchcock, with whom Chandler would also worked acrimoniously and unsatisfactorily on Strangers on a Train in 1950.) Of course, writers Graham Greene, Stephen King, Kurt Vonnegut and Hunter Thompson have also popped up on the big screen. But their appearances were all recognised at the time. That is not the case with Chandler and Double Indemnity, one of the earliest appearances in a Hollywood classic of a notable novelist.

Chandler himself was a reclusive sort. He gave very few interviews of any description (apart from, famously, a radio chat with fellow novelist Ian Fleming, in London for the BBC in 1958). There is no television footage or film of him at all, apart from a snippet from a home movie. His preferred form of communication was the letter, although there is no apparent reference in any of his epistles to his appearance in front of the camera during the making of Double Indemnity. Wilder never mentioned it either in any of his many interviews about the film.

Despite all of this, there he is, 16 minutes into the movie, sitting outside an office as Fred MacMurray walks past. Chandler glances up at MacMurray from a paperback he is reading, in hindsight a rather obvious clue about the true identity of this extra. Sadly, it is impossible to determine what the book is as the film briskly moves on and Chandler vanishes.

Who knows whose idea this magical little appearance was? Maybe Billy and Ray, in one of their more amicable moments, thought it would be a nice joke. The reasons are for the cameo are unlikely ever to be known. But, somehow, it is enough that we have this little magic moment, with Chandler secretly inscribed into the film. And how fitting that this piece of knowledge should be uncovered in the 50th year since Chandler's death.

• Adrian Wootton is appearing on 17 June at Cardiff Chapter, on 19 June at BFI South Bank and on 1 July at Broadway Media Centre, Nottingham to give illustrated talks on Raymond Chandler's life and career