They would join in their own group and plot minor infractions and then do so as a group thinking they were fooling us.
The funny thing is the Americans caught them secretly and never did anything about it. The Japanese would have been humiliated it they knew that we knew they were not as clever as they thought.
Get them in group, however, and they reflexively think and act in a group. They pride themselves on their ability to alter their opinions and behavior to adjust to group dictates. This quality is called makoto, which translates as "sincerity," but, like so very many other things in Japan, the meaning in Japanese is the precise opposite of what it means in English.
For them, being sincere doesn't mean sticking by your principles or being devoid of guile. It means suppressing your self for the sake of the group and acting accordingly.