Hello davenmidtown, the triangles give an indication of increasing or decreasing traffic to a particular hub.
The red triangle points to increasing traffic, a blue triangle indicates decreasing traffic.
They indicate relative increases to the hub itself and not whether the hub is being recorded in search engine pages.
I have found that traffic generally increases if the hub is highlighted in a particular hub topic front page. So for instance if I have an increase in a hub on blindness, and I look to the visual disorder page often one of my hubs may be being highlighted.
Hope this helps.