I think Maelochs made a valid point.

The same thing happened to me a few years ago. I had this bike for almost a year with a cable lock until I decided to leave it at the train station. Big mistake. Mid of the second week it was gone.

With hindsight, cable lock should never be used to leave a bike for long periods of time, especially if these periods are consistent. It's like an open invitation for potential thieves - hey take me! My guess is that the bike was taken by someone who came to the office for some reason (a courier?), saw the cable lock and came back the next day with a couple of his buddies.

live and learn, as they say

let me ask you this: you pop in in a shop for two minutes to buy a soda and leave the car key in the car, and someone steals the car.
Is it your fault they stole the car?