As mentioned earlier, the amount of energy Shin had or used was definitely less than that of Raka and Terran.
Even if they were cursed and most of their powers were sealed, they still had the power inside them.
As for Shin, no matter how hard he tries, right now, he cannot defeat Terran and Raka if they were to regain their peak form. So the Dark Elves should have been able to see the amount of energy that Shin had clearly.
But yes, they failed to see and the reason was quite simple actually.
Shin had the energy for sure, but the energy wasn't stored inside his body. It was entirely and completely stored in the system.
If the energy was indeed inside Shin's body, then his body wouldn't be able to handle that much amount of energy. No matter how you look at it, for Shin's age, that much energy is abnormal.