At the rooftop section of level 2_1. Two ways of getting it:
1.- Cheating. Enabling cheats on the console and noclipping towards it
2.- But we are here for the "legit" way. It is the usual exploit speedrunners use in their business. Pick a physics object, get on top of it, and alternate the use key and jump key. If you do it right (unlike me), you will start to float upwards into the air each time you do it, allowing you to reach the top of the building. Then go along the ledge and voilá!