telescopic control joint

This is a movement joint of a vertical mullion of stick built curtain wall. The picture shows a missing caulking between gasket and mullion splice (The daylight is visible in the gap on the photo). This deficiency constitutes the popular leak origin.
The leak of this kind is very difficult to trace, because water travels long way before reveals itself inside. The origin location is deeply concealed inside a spandrel, behind firestop and soffit at the slab level.