Sadly, if you live in London or probably any major city, bike theft is a bit of a problem. It does seem like there is a growing and worrisome new dimension to this all.
Up until recently bike theft involved you leaving / locking your bike upon somewhere to discover that someone had managed to break the lock, chain - or whatever else was used - to secure your bike and made off with it. There is of course a whole industry devoted to bike locks that compete with each other to be top dog.
The new dimension is that there are now reports of gangs (for want of a better expression) targeting cyclists as they are riding their bikes. Some of these are on scooters and some all but ambush cyclists going by. They use the shock value of actually knocking someone off their bike to then make away with it. In addition there have been reports of a considerable amount of violence used to achieve this. Some thefts of this nature take place in broad daylight.
The Brompton is an expensive item and therefore very attractive to those who would want to steal it. I am sad to say that there are plenty of people out there who would be more than happy to buy one second hand, with a shady provenance, if it meant they could save a few pounds!
So, what can you do? As far as someone stealing your Brompton if it is locked up somewhere, I just don't understand this when you can simply fold and take it with you? In fact if I cannot go into an establishment with my folded Brompton, I turn round and leave. (It is perhaps strange to note that the locations you would expect a folded Brompton to be unwelcome are the ones that are the most accommodating).
As far as riding your Brompton along quite happily and a couple of shady types trying to take it off you on foot or on a scooter, you would have to weigh things up very quickly. My 9+ years of Judo and Karate in my youth taught me that it is better to take flight rather than fight.
Some people go down the GPS tracking route but these have limited use in the real world. Firstly they are small and discrete and as such the battery powering them has a limited life, so even if you can locate your bike you probably won't have long. I am sad to say that I have my doubts as to whether the police would be truly interested or inclined to act upon you showing them the location of your bike on a mapping app on your smartphone.
If you have a Brompton you can register your frame and serial number with them which is a start. If you are in the UK there are also registering schemes such as 'Immobilise' or 'Bike Register.'
As I mentioned the police might not be that interested in the big scheme of things and you might have to do some of the detective work yourself. Ebay, Gumtree is where you might want to look. Telling your friends and family to look might also help. In fact a few years ago a Brompton that was stolen from outside a location was tracked down by an eagle-eyed Brompton owner.
Terrible that this goes on but it exists. My advice for what it is worth is not be try and be a hero. Get away - with your bike - if you can, but if this isn't possible (as hard as it may be) let it go. The bike can be replaced. You can't!!
I welcome your thoughts on this one readers so please leave a comment.