Can a leak be something other than the façade?
Yes. ASTM E2128, the standard guide for evaluating water leakage of building walls, deliberately excludes leaks from roofing, condensation, plumbing and open windows, because these are common and are often mistaken for wall leaks. Check air-conditioning drains, pipes, roof outlets and condensation on cold surfaces before testing the façade.
From the RRD Field Guide, checked against the sources cited in Façade and Curtain Wall Water Leakage: How to Diagnose and Fix the Root Cause. General information, not advice for your project.
The fuller picture
Façade leaks are diagnosed by mapping when and where water appears, ruling out non-façade sources, then reproducing the leak with controlled water tests: AAMA 501.2 nozzle testing for curtain walls and ASTM E1105 pressure-chamber testing for windows and walls. Test from the bottom up, isolate one joint at a time, and repair the proven path rather than re-sealing everything.