A major security alert involving the Queen in Northern Ireland was prompted by a Tory MP's child on a visit to a Belfast pizza restaurant.
Sasha Swire, wife of former junior minister Hugo Swire, has told how she took their daughter Siena for a bite to eat before she was due to meet the monarch.
But the little girl left behind her black notebook in which she had detailed all the arrangements of the royal visit, because she had been copying the protagonist of her favourite children's novel, Harriet The Spy.
It led to an alert that saw an anti-terrorist squad seal off the streets to retrieve the notepad so that it did not fall into the hands of dissidents.
Sir Hugo became a Minister of State for Northern Ireland in May 2010, and five months later in October the Queen made a two-day visit, during which Siena was to present a posy.
Mrs Swire revealed: "She is still physically attached to her little black spying book and clings on to it, next to her heart.
"God, what a fuss it all is. Siena and I are sitting quietly in the flat the night before the arrival when about nine security officers with all kinds of terrorist-busting machinery with flashing red lights come over to check the place out - dogs, even, jumping onto the sofa, sniffing around in my knicker drawer. Siena is of course scribbling it all down, hiding behind the sofa.
"In the morning I take Siena into central Belfast for a pizza before the Queen arrives at Hillsborough. In the car back to the castle I say: 'You have got your little black book, haven't you?'
"Siena knits her brow and purses her lips, then rummages through her bag: 'I think I left it behind in the pizza restaurant.'"
She details the drama that unfolded in her soon-to-be-published book Diary Of An MP's Wife: Inside And Outside Power, and says she asked her daughter if she had written anything about the Queen in her notebook.
Her daughter told her: "Yes, I did... what time she was coming in, what door, what time her plane lands. Those sorts of things. What her policemen are called."
The revelation led to the full-scale security operation to close down Belfast city centre to retrieve the notebook.
Mrs Swire said: "I almost drive the car into the hedge. Rummaging manically through my own bag to find my phone, I ring H's private secretary, Deborah. 'I think we have a problem.'
"I explain what's in the book, most notably details about the Queen's upcoming visit, but also all David (Cameron, then Prime Minister)'s own security detail, which Siena wrote down from the cards they gave us back in the summer. Deborah says: 'Leave it with me'.
"Within minutes of the call, it feels like the entire anti-terrorist squad has been dispatched to central Belfast and a rolling pandemonium ensues.
"Shoppers are shoved aside, streets are roped off, routes are retraced, a phalanx of policemen moves like a tide into the pizza restaurant, where I expect the purely innocent Italian waiter who served us earlier raises both his hands, surrendering to the crime he has absolutely no idea he committed.
"'Got it!' says one of the policemen. Phone calls are made. The squad scoops up the team, armoured vehicles are mounted and the little black book is winging its way back to Hillsborough, where we decide that it is best we shred it."
Mrs Swire said she only started keeping a diary when her husband (60) was "honoured" to be made a minister after the 2010 general election, and kept it until he stepped down as MP at the 2019 general election.
In them she also takes a swipe at the decor inside Stormont, writing that it failed to make a good first impression upon her husband. She said: "He is met by security then taken straight to his armoured car, then on to Stormont House, where he meets all his staff and is shown his private office, which he tells me is 'the size of several tennis courts'.
"Stormont has recently been refurbished in the most tasteless of fashions by the female head of the Northern Ireland Office, Hilary Jackson.
"H has accommodation there if he wants it but decides to take up a berth at Hillsborough Castle instead. Much more his style."
Diary Of An MP's Wife: Inside And Outside Power by Sasha Swire is published on September 24.