I dont think we have any idea what lock it was or how it was taken. Having said that, the bike is over 30 years old. You wouldnt think it would come high on the bikeknappers list.
The old guy has done pretty well keeping it safe all thatt time.
It was a 14 / 30 year old Thorn with a BMW badge tacked on the front being ridden in Thanet, if it had a Kryptonite New York on it I will personally buy him another bike.
I'd imagine the thieves have been too busy nicking unlocked/barely locked student bikes but spotted it as an opportunity while at the hospital for something else.
They don't even have to be that clever to take the lock with them, the one who did it with mine in Greenwich was known to police (apparently) and fully caught on CCTV during the theft. The bike was never recovered of course.