What happened to my drywall... insects?
Yes - termites will do that. They will go anywhere there is moisture, and eat any cellulose products along the way. They will tunnel through Sheetrock and plaster. They will even eat the paper under the paint, and tunnel through the paint, to any wooden furniture or cardboard boxes, etc, against the wall.

If the moisture issue has been corrected, and/or the house treated, then they have most likely vacated or died. That doesn't mean that they won't come back though - usually entire neighborhoods are infested, and they are managed, but never completely eradicated.