What is 'artistic thieves'-kitchen'?
GRANT had just come back from Hampshire, where a case had ended unhappily in suicide, and his mind was still reviewing the thing, wondering how he might have managed things differently to a different end; so that he listened with only an ear-and-a-half to what his superior was saying to him until a familiar name caught his whole attention.
'Salcott St Mary!' said Grant.
'Why?' said Bryce, stopping his account. 'Do you know the place?'
'I've never been there, but I know of it, of course.'
'Why of course?'
'It's a sort of artistic thieves'-kitchen. There's been a migration of intelligentsia to the place. Silas Weekley lives there, and Marta Hallard, and Lavinia Fitch. Tullis has a house there too. It isn't Toby Tullis who is missing, by any chance?' he asked hopefully. (Chapter 8 To Love and Be Wise, http://gutenberg.net.au/ebooks08/0800551h.html#ch08)

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Ben has given you the right answer.

This expression has nothing at all to do with thieves or or with stealing! Just think of it as a set phrase - as a kind of 'den' where lots of artists have gathered together away from other people.

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"A thieves' kitchen" is a hideout for thieves.  A hideout is where people go if they don't want to be found.

The sentence here means, "It is a place favoured by artists."   It has nothing to do with theft.  "A thieves' kitchen" is used figuratively here.  The next sentence "There's been a migration of intelligentsia to the place" elaborates on this, explaining what kinds of people have moved here to live.

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In this context I believe the author is comparing the place (salcott st mary) with a kitchen for artistic thieves. He is saying that they would gather everything they need at this place, much like a kitchen. The thieves are not stealing art, they are robbing people in creative or memorable ways. In this way they are artistic. Their theft is their art.