Was there's moisture in the place you store it for two days?
Nope, dry and cold.
Maybe crap from the road or something. Or rain running down the hose into the caliper and washing the dirt down onto the disc.
The caliper does look dirty.
@Murphy's_Law started
London Fixed Gear and Single-Speed is a community of fixed gear and single-speed cyclists.
Was there's moisture in the place you store it for two days?