I've seen "structure destroyed" before, but not in logs. I believe they are the generic structures at bases that are basically eye candy that don't really do anything. If something can be destroyed that actually does something, it's named: guns, barracks, fuel, ammo, hangar, etc.; and that's what is recorded in the logs. But...it's only a guess.
Structures are part of a base's object group, and not a single object that is generally added to a terrain when it's being created.