AET requires the component to be loaded in some manner that will cause the material to yield or the crack tip to propagate, thus emitting a high frequency squeal that can be detected by the transducers. If the weld is located such that it is not loaded, no sound will be generated and no signal will register on the display.
A crack or localized yielding in the vessel wall will generate noise that can be detected by AET when the vessel is pressurized. However, a bracket on the inside of the vessel may not be loaded by pressurizing the vessel, thus a “bad” weld would not produce a signal and will go undetected.
Best regards - Al