The Defiant Agents — Andre Norton

No windows broke any of the four plain walls of the office; there was no focus of outer-world sunlight on the desk there. Yet the five disks set out on its surface appeared to glow -- perhaps the heat of the mischief they could cause ... had caused ... blazed in them.
But fanciful imaginings did not cushion or veil cold, hard fact. Dr. Gordon Ashe, one of the four men peering unhappily at the display, shook his head slightly as if to free his mind of such cobwebs.
His neighbor to the right, Colonel Kelgarries, leaned forward to ask harshly: "No chance of a mistake?"
"You saw the detector." The thin gray string of a man behind the desk answered with chill precision. "No, no possible mistake. These five have definitely been snooped."
"And two choices among them," Ashe murmured. That was the important point now.
"I thought these were under maximum security," Kelgarries challenged the gray man.
Florian Waldour's remote expression did not change. "Every possible precaution was in force. There was a sleeper -- a hidden agent -- planted -- -- "
"Who?" Kelgarries demanded.
Ashe glanced around at his three companions -- Kelgarries, colonel in command of one sector of Project Star, Florian Waldour, the security head on the station, Dr. James Ruthven....