Skier triggered slide

No signs of instability while breaking trail or descending slopes up to 35 degrees. New layer of surface hoar on the snow surface today. We observed a group of 3 trigger a size 2 slab avalanche while ascending a steep, alpine slope near Ursus Major. No one caught. They said it started as a D1 and stepped down. They also said they had stable pit results and no signs of instability enroute.

Unsure if it was a wind slab or a persistent slab. Viewed from afar. Dimensions are estimates