According to the Association of Chief Police Officers Vehicle Crime and Intelligence Service, the Ford Transit is the most common vehicle to be stolen.
They say because the Transit is about in such large numbers, the market for stolen parts is huge, which makes them very popular with criminals. The vehicles are usually stripped for parts which are sold on the internet, whilst the bigger bits are then sold on as scrap metal.
Ford has a research and development centre which works towards preventing its vans being stolen, in fact they point out that a modern Transit can’t be driven away without the keys, which suggests that owners and fleet managers, should be very careful how they store the keys and who has access to them.
Clearly, the way Transit is used makes it more vulnerable, as often, they are left unlocked, perhaps during a door to door delivery, or when collecting building materials, or simply sitting on site. As a lease company, we have experience of some of the silly ways they get stolen. Just looking at a couple of events, we had one where our customer parked it up and left the van unlocked with the keys behind the sun visor, returning to find the van had gone. The oddest was a driver delivering some goods to a building site, who walked to the back of the van to open the doors (leaving the keys in the ignition, as you just might do as you were not going to leave the van), but as he opened the rear doors, he saw someone sitting in the drivers seat, the van instantly started and drove off, leaving the delivery driver running down the road to try and catch it. Needless to say, he never saw that Transit again.
So short answer is as with any vehicle, always take the keys with you, and always lock the vehicle if you leave it.