I can tell you when I am wandering around I get trap warnings from various party members. I would take it to mean that every party member is testing perception as you hit detection radius.
This was my understanding as well. One character with high perception is all you need, but the detection is dependent on that characters radius. So if you have someone with high perception and you suspect traps in the area, it might be good to keep them in the front of the formation, or move slowly so they have time to detect things near your lead character.
