The carpenter ants in this area are quite large, so I am guessing that this is not what they are, but I don't know what there is in your neck of the woods. It would be worth checking. They don't eat wood, unlike termites; they just dig out burrows in it. However, they dig BIG burrows. Here is a shot of a only a small part of the damage one infestation did to our house:
We had to remove siding for a vertical distance of about 10 feet to expose all of the damage, and we had to replace structural lumber.
All of which is to say: I would want to check out the area around where you found the swarm, if you don't have any creatures left to examine.