Inside an illusion, everything wasn't real. And so even if he tried to use his spirit sense to see through everything, he wasn't actually using his spirit sense in full power, or even half of it.
If he managed to use his spirit sense in full power, then the entire illusion would be broken. And so, he built another array, one that could augment his spirit sense to some extent.
He knew his breaking array would cause a change, subtle and minute that could easily be missed, happening somewhere away from him and he'd not notice it.
He built the second array to make sure he'd miss a single thing. He stood in the middle of that array and activated both.
*Sizzle!*
"As expected…" he saw a little change, some sort of a flame that appeared just one kilometre to his northwest. The fire spark just appeared and vanished in a blink of an eye, yet he managed to capture it.
