You can "unlock" the Café Théâtre after finished Sequence 2. The Cafe is located on the east Side of the Paris Map. Here a Picture (red Circle)
Climb up or walk inside the Cafe to the Balcony from Arnos Room, go closer to the railing and Arno leans against. Rotating the camera a little up or / down if it is necessary and the Achievement unlocks.
The Video shows as first
Its the same Location but one Floor down.