I did not realize this but am posting the response so that you all are aware. This way you will check all your buildings when they are ready for collection.
Inno's response is as follows:
The cubes indicate that there are rewards other than supplies or coins to be collected. Many buildings will display a box when not motivated.
Inno's response is as follows:
The cubes indicate that there are rewards other than supplies or coins to be collected. Many buildings will display a box when not motivated.