with local testing, it seems to me that the “low” sensitivity setting on the panel actually results in the lightest shock triggering an alarm (rather than “high” sensitivity setting), which is the opposite …
The lower the threshold, the higher the sensitivity. If the Low setting is too sensitive change it and test until you find the best one of the three (Low, Medium and High).
…is tapping the window locally and looking for the red LED to illuminate an accurate method of testing?
DSC recommends gently striking the surface that the sensor is mounted on and gradually increasing pressure until the sensor triggers. Localized testing with panel programmed sensitivity settings would be recommended.
While the LED should light up red for 2 seconds when shock is triggered, ultimately, you are looking for the sensor to trigger at the panel.