The bike was locked to a bike rack in a gated and locked apartment courtyard. The rack was not visible from the alley or the street, so whoever stole the bike needed to know it was there first. The gate can be jumped, so it's also possible that someone did that.
Protect your bike – get a security protection label!
Apply the protection label to your bicycle and instantly reduce the risk of theft by up to 50%! Moreover, if the bike is stolen, you increase the chance of its safe recovery by up to 70%.