I had a bike like that (quote a friend of mine after borrowing it for someone else: "That bike is plain dangerous!") stolen after I left it on a bike rack on the sidewalk for a few days, since I needed it out of the flat. And it was locked - but with a flimsy lock.

The best deterrent I found? No air in the tires. Bike will remain there for weeks.

Had a loved bike stolen from me on the bus stop since I had no time to lock it. My brother found it in the schoolyard on his way to work with empty tires (tube lost air very slowly).