The papers also appear to give insight into other parts of the process
A document has been photographed outside Number 10 Downing Street, appearing to reveal tactics on Brexit.
Embarrassingly for British Prime Minister Theresa May and Brexit Minister David Davis, the handwritten notes on the document clearly say: "What's the model? Have your cake and eat it."
The latest victim caught carrying an unguarded document in Downing Street is thought to be Julia Dockerill, who works for Conservative Party vice-chairman (international) Mark Field.
Ms Dockerill and Mr Field, MP for Cities of London and Westminster, were in a group of four walking from Mr Davis' DExEU (Department for Exiting the EU) office in 9 Downing Street up the street to Number 10.
The notes, which filled a whole page of an A4 notebook, suggest no single market for the UK after Brexit, no "Norway-style deal" and accuse the EU's Brexit negotiators of being "difficult".
But within minutes of the leak, Number 10 dismissed the document. "This is not a government document and it does not represent the UK's position in relation to Brexit negotiations," a source said.
But the notes are revealing because they address some of the most contentious arguments over the UK's Brexit negotiations, on which Mrs May has refused to give a running commentary, including:
Mr Field's role as vice-chairman (international) includes liaising with the Tories' sister parties on the centre-right and acting as a link between the Conservative Party and its MEPs.
Besides working for Mr Field as his chief of staff, speechwriter and senior researcher, Ms Dockerill is a Conservative councillor in the East London borough of Tower Hamlets.
She is the latest politician, public figure or party official carrying a sensitive document to be caught on camera by freelance photographer Steve Back. The most recent, in September, was on grammar schools.
It is reported that there is a sign on the Department for Exiting the EU exit doors reading: "Stop! Are your documents on show?".