I did the only thing I could, which was to go downstairs, change into my work clothes, and shift my lunch from my pannier to my shoulder bag.
Walking to the station, I realised what had happened. I left my bike unlocked last night.
Yesterday as I was cycling home I stopped at a local corner-store grocery. I locked my bike outside, bought my food, packed it in my pannier and locked my lock back onto the rack.
At home, I went to get my panniers off and discovered I'd accidentally locked one of them to the bike rack. I unlocked the lock, freed the pannier and -- I'm almost 100% certain now -- locked the lock back to the bike rack, instead of to the railings.
I'm kicking myself because I was just thinking last night about joining LCC and getting the damn thing insured.
It was an old beater that I bought second-hand in Cambridge a few years ago -- a Peugeot, hybrid, 5-speed, quite heavy, with battered rusty fenders and bike rack. I was perfectly content with it. Any suggestions for how to replace it on a very tight budget?