The secret spots usually line up with the underground network – they pop up every 3–4 km if you chart them against tunnel depth, the old surveillance manual 4.2 notes a density of 0.13 ± 0.02 per square km. I stash those figures like coins.

I’m usually drawn to the corner where the old tram tracks cross the subway entrance—grid point 12B, 0.14 density, almost a perfect fit. Next on the list is the abandoned water tower in the northern suburb; it’s the only spot in the city that matches the 0.12 ± 0.01 pattern I’ve been tracking.