Artemis- Gel Pack in Toy Monkey

She quickly selected the command sequence for a thermal scan, and the helmet infrared detector analyzed the electromagnetic radiation coming from inside the aircraft. There was some waffle from the solar panels, but the scan isolated an orange blob in the pilot’s seat.

One passenger only. The helmet’s biometric reader conveniently identified the pilot as Artemis Fowl, and dropped a 3-D icon over his fuzzy figure.

“One passenger,” murmured Opal. “Are you trying to decoy me away from the house, Artemis Fowl? Is that why you fly so low?”

But Artemis Fowl knew technology; he would anticipate thermal imaging.

“What do you have up your sleeve?” wondered the pixie. “Or perhaps up your shirt.”

She magnified Artemis’s heart and discovered a second heat source superimposed over the first, distinguishable only by a slightly cooler shade of red.

Even at that desperate moment, Opal could not help but admire this young human, who had attempted to mask the lemur’s heat signature with his own.

“Clever. But not ingenious.”

And he would need to be ingenious to defeat Opal Koboi. Bringing back the second Artemis had been a neat trick, but she should have caught it.
...

Artemis glanced down to the V of his jacket. The furry head was gone.

Dropped on the Little Sister. Waiting for Opal.

This was confirmed by a sudden shriek of delight from behind. Artemis turned slowly—and with great effort to face his enemy. It seemed
as though he had been fighting her forever.

The pixie stood atop the sea stack, almost dancing with delight. Artemis could see a small furry figure splayed on the plateau.

“I have him,” Opal cackled.“With all your genius! With your big bursting brain! You dropped him! You simply dropped him!”

Artemis felt a throb build in his shoulder. In a minute, there would be worse coming, he was certain of it.

Opal stretched two hands toward her prize. “He is mine,” she said reverentially, and Artemis swore he heard thunder in the distance. “The ultimate magic is mine. I have the lemur.”

Artemis spoke clearly, so his words would carry across the divide.

“It’s not a lemur,” he said. “It’s a monkey.”

Opal’s smile froze, all tiny teeth, and she grabbed what she had thought was Jayjay. The figure was soft in her hands.

“A toy!” she gasped. “This is a toy.”

Artemis’s triumph was dulled by pain and exhaustion. “Opal, meet Professor Primate. My brother’s plaything.”

“A toy,” repeated Opal dully. “But there were two heat sources. I saw them.”

“Microwave gel pack stuffed inside the foam,” explained Artemis.

“It’s over, Opal. Jayjay is in Haven by now. You can’t get him. Turn yourself in, and I won’t have to hurt you.”

-The Time Paradox, Chapter 15