Carpenter ants are mostly looking for moisture... so the wet cardboard was probably what they were after. They are a great study in random behavior on a massive scale in finding food/water. The scouts travel around laying a scent trail in random patterns around the main nest. When they find food or water, they immediately return to the nest. Eventually enough of them have returned from a consistent source of food or water to the nest to leave a strong trail for others to follow. That probably happened to your sail box and you have the ones that were just there temporarily drinking... kind of like sailors randomly sailing a windward-leeward course and ending up at the bar at night.

To eliminate them, you really must find the main nest. Probably in a nearby LIVE tree... they depend on the tree drawing moisture out of the ground and up the trunk for the nest to survive. If not a tree, find any constant supply of moisture... leaking pipes, downspouts into foundations, etc.