It had always been a point to take note of, L had thought, that Light's behavior was unusually innocent. That is, in all the time they had been watching him, not one action of his seemed even slightly suspicious. Nobody could incidentally behave that way. Anyone could have flipped the channel to the news during dinner, any family would have discussed what they thought of their overworked father, any teenager might have put a piece of paper in his door for privacy. But the television only showed mindless programming, the conversation topic stayed innocent, and the security was surprisingly justified. Suspiciously innocent.
L might have thought that Light knew he was being watched. But there was no way he could know. Yagami-san certainly didn't tell him- there was no opportunity for him to have, even if he would betray the investigation, which L doubted. Light's insignificant alert system had been replaced exactly, so it couldn't be that he would suspect anything. Maybe he really was that innocent.
And maybe not. Most likely not. Even if Light were not Kira, he was a prodigy. He would likely have some habit that set him apart from everyone. But the only thing that seemed different was his enthusiasm for studying. He had every other habit that a teenage boy might have. Light behaved as if he was being watched at every moment, but he didn't want to alert his watchers.
L watched as Light continued with his homework. The textbook sitting next to him said mathematic; the actually work was blocked by his body. If L honestly believed he knew of all the cameras and could figure out all their blind spots, he would be highly suspect of that piece of paper. But there was no way he could know, so L only desired strongly to see what the teenager was working on.
In the end, another day passed and there was no change. No suspicious behavior. No sleep-spoken confession of being Kira. It frustrated L to no end, because he knew something wasn't right, though he couldn't explain it. It just made him suspect Light further.
The day before the surveillance was planned to end, L saw Light come home from the store, not with a supply of magazines as he had the first day, not groceries, but a large container of refillable pencil lead. It was the first odd thing that had happened.
"Yagami-san," L addressed the police chief, "Why did your son buy so much pencil lead, do you know?" He took a bite of cake as he waited for the answer.
Light's father looked taken aback. "Why? For school, I think. Why else would he?"
L shrugged loosely from his position crunched up in his chair. "It is something unusual, to buy so much. I thought perhaps he might have some other reason for doing it."
"Like?"
"Like, I have seen it used as a more in depth security measure. I admit, though, there are so many obscure methods of detecting whether a room was entered that we can't check for them all. I just find his excessive amount of pencil lead interesting, when I didn't notice him consuming it too quickly before."
Yagami was quick to jump to defend his son. "You think Light is Kira because he bought school supplies! If he had known his room had been invaded, don't you think he would notice it? If he knew he was being watched, wouldn't he be livid about it!"
L blinked at the older man. Poor Yagami-san. He didn't realize he was incriminating the son he was trying to defend. "He would be livid, unless he was trying to make sure we didn't know he knew. That way, he could prove himself innocent. And why would he try to prove himself innocent if he wasn't guilty?"
"There is no justice in that!" The chief exclaimed. "Knowing he was suspect and being innocent would make him want to prove himself also! He wants to help us catch Kira, and so, in his innocence, he would want us to dismiss him as a suspect so we could find more likely candidates and not waste our time on him!" L just shrugged. If he had the personality to be innocent, Light would likely confront his father over the privacy invasion. Or at the least, inform his mother and sister. If he had the personality to be Kira, he would plot how to convince his watchers that he was innocent. L no longer saw any other possibility than that Light was Kira.
Yagami walked in to his own home after so long of being away. His family rushed towards him, and he was rewarded with hugs from his wife and daughter, as well as a fond, welcoming smile from his son. Over dinner, however, what L had said that long time ago stayed with him. He only came home once in a long while, but somehow, he wanted to prove his son's innocence to himself.
Walking up to Light's room, Yagami knocked softly. He heard the click of the lock as his son opened the door. He inhaled slightly. Since when had Light locked his door? It was somehow different to see it in person then to watch it on a camera. He also glanced surreptitiously at the floor for broken lead. Light smiled at his father.
"Studying?" Yagami asked. Light nodded.
"I need to keep my knowledge up for university," he explained. His father nodded proudly.
Then he glanced the stack of lead containers on the boy's desk. "Light..." he asked trailing off before asking the question. "Why do you have so much pencil lead? Do you write that much?"
Light laughed at his father's random question. "I had been meaning to talk to you about this, but you haven't been home. I'm doing a study of the appearance of fingerprints in different substances. I haven't really done much work at home on it yet, but I intended to start with crushed pencil lead because it still has much of the clarity of ink, except with a slight variation. However, before I moved on, I wished to ask you what substances with which you think I should proceed. After all, I chose this topic since I am interested in becoming an officer like yourself."
Yagami muttered something; he didn't remember what it was, he was so filled with relief. His son had a reason, you see, L? And there was no lead in that door, even if he had been lying. Light was innocent.
Light closed and locked his door. He glanced at the Death Note on his bed and grinned, putting the piece of lead in his pocket. He had always known they would notice. And he had always known they would never take note.