This remained a mystery to me until only a few years ago when a fellow Bromptonian informed me that they had probably opened and closed it by simply using a Bic Chrystal pen! There was a silver lining to my bike being stolen insofar as I was gifted a Brompton that I used for a couple of years before gifting it to someone else.
I recall this story as in the news recently was the story of Bedfordshire Police being overwhelmed at discovering a haul of over 100 stolen bicycles. I am sure that many of the owners of these bikes had locked them up with all sorts of locks - hopefully better than the one I used all those years ago!
Since owning a Brompton I have never locked it up anywhere unattended. It is not something I do. If I go to a shop and find I am not allowed to enter with my Brompton folded up, I just turn and walk straight out. Of course in the current climate I would expect a Brompton to be barred from entry for obvious reasons.
In my experience the more exclusive the shop the less fuss is made of you carting in your Brompton. The same cannot be said of some of the more well-know establishments found on every high street.
So, I will continue to take my Brompton with me everywhere I go, not leave it unattended and certainly never let it out of my sight.
Stay safe out there people!!