According to the Garmin website, the Garmin GPSmap 66 includes an SOS beacon:
These devices include an LED flashlight and SOS beacon that can be used to signal for help.
They are thin on details, and Google Search just turns up the exact same formulation on different websites. What does this SOS beacon actually do? Is it anything like a PLB, is it an audible/visible alarm, or something different from either?