It can and does occur at all hours of the day and at all locations. I've seen broken glass from the cottonwoods all the way up to cement slab. Some of the worst places are Lickman, Browne, giesbrecht, Bergman. Always broken glass at those spots. Oddly enough, the upper seems a bit safer.
Just as I hit post, I realized something.... I do not see nearly as much broken glass in the upper. Maybe this is because the culprits are working in pairs or groups and the communication issues up that way prevent them from using spotters or lookouts.