The Scots interrogative phrase who stole your scone? is addressed to someone who looks glum.
Here, the noun scone denotes a light plain doughy cake. Interestingly, this noun, attested in the early 16th century, was originally Scots.
I have found an isolated 19th-century occurrence of who stole your scone?, used as the title of a comic song, in the following passage from the account of an amateur concert, published in The Falkirk Herald and Linlithgow Journal (Falkirk, Stirlingshire, Scotland) of Thursday 11th March 1869—but it is impossible to know whether this represents an early use of the phrase, since the words of the song are unknown:
The comic department was ably sustained by Mr Robt. Stewart, of whom we have to speak in the most favourable terms. His various efforts were loudly applauded, and his humorous rendering of such songs as “Wha stole your scone?” “Fifty-and-twenty shillings a week,” and “Fifty years ago,” provoked the hearty laughter of all present, and afforded great amusement.
The earliest occurrence of the phrase that I have found is from Tales of Tilda—Conductress, by one William J. Taylor, published in The Port-Glasgow Express (Port Glasgow, Renfrewshire, Scotland) of Wednesday 25th January 1928:
It was a quarter past two on a Thursday afternoon, and Tilda McKay, the most popular conductress of the Firstoval Bus Company, was proceeding down Queen Street on her way to the garage, when she met a friend called Bella McCrae.
“Hello, Bella,” said Tilda. “What’s the matter? You don’t look too happy. Who stole your scone?”
The phrase then occurs in this passage from The Land Of The Leal (London: Collins, 1939), by the Scots novelist James Barke (1905-1958)—Jean and her family are moving to Glasgow; a cab is taking them to the train station:
Jean could bear the strained silence no longer. She shifted uneasily on the creaking leather.
‘God o’ Glory . . . hae you a’ [= have you all] been struck dumb? Andy: get your head up out o’ your feet.’
Andy lifted his head slowly and looked at his mother. His eyes smouldered with resentment.
Jean noted the look and fear gripped her. She looked at David: but David dropped his eyes. She wondered what was wrong. Why were they afraid to speak? She turned sharply on Nancy.
‘I suppose somebody’s stolen your scone tae [= too]?’
‘Oh, you’re a’ sae [= all so] damned pleasant and agreeable.’
Toddy Beattie mentioned and explained the phrase in the column Vancouver Day by Day, published in The Vancouver Sun (Vancouver, British Columbia) of Wednesday 27th December 1950:
“Who stole your scone?” is a good Scots expression, which, translated roughly, means “Why so glum, chum?”
In Tea at Miss Cranston’s: A Century of Glasgow Memories (London: Shepheard-Walwyn (Publishers) Ltd, 1985), Anna Blair quoted one Kate Thomas as recollecting the following about children’s books and games:
But you couldnae aye be readin’ books or playing tiddlywinks. There were times, when you were wee, that it was just your teddy or your doll that you wanted . . . when someb’dy’d stolen your scone or give [sic] you a cuff on the ear.
A drama broadcast on BBC Radio 4 on Thursday 23rd April 1992 and on Monday 28th March 1994 was titled Who Stole Your Scone?. The following, for example, is from the radio programmes published in The Guardian (London and Manchester, England) of Thursday 23rd April 1992:
Who Stole Your Scone? Drama by Marion Brechin about the love between an elderly and confused mother and the daughter who cares for her.
Marion Brechin was apparently the nom de plume adopted by Marion Pitt (née Marion Davies – Renfrew, Renfrewshire, Scotland, 1939–London, England, 1995), social worker, playwright and (under the name of Mary Black) columnist for The Guardian (London and Manchester, England). The following is from her obituary, published in The Guardian (London and Manchester, England) of Wednesday 15th November 1995:
She kept saying she would write a play based on the last days of her mother. She put it off, as people do, but eventually she did. Who Stole Your Scone? was performed by the Women’s Theatre Group at the ICA [= Institute of Contemporary Arts], London, in 1990 and later became a play on Radio 4.