Kakashi felt a twinge of apprehension at the look of hope that rose in Naruto's eyes when he mentioned the possibility that the Demon of the Mist hadn't actually died. He knew that Naruto would be willing to fight the man if it came down to it, but he suspected that Naruto would be unable to kill this particular opponent were it necessary and that would be one weakness Momochi Zabuza would exploit in an instant.
He knew it would come down to him fighting the swordsman while Naruto and possibly Sasuke fought his suspected accomplice, but he feared that Naruto would do something foolish to protect his recently discovered family member and possibly get himself killed. Or, Naruto would get himself hurt while trying to cause Kakashi himself a great deal of pain and misery after he killed Zabuza. Should Zabuza's death and a subsequent attack fron Naruto come to pass, he would be forced to defend himself. Mainly because of what would happen to the boy if he harmed any of his own comrades for any reason whatsoever. Naruto was his father's son when it came to what was his, or what he considered to be his whether or not it was actually his. That had become patently obvious after what Naruto had done to Mizuki when the traitor had injured Iruka.
Kakashi had about a week to think about both scenarios though, and therefore had a week to come up with a viable plan. He had no desire to harm his sensei's son or allow harm to come to him, nor did he have any particular desire to defend himself from a pissed off Naruto who was intent on ripping his balls off for killing what was probably his only surviving relative considering the political climate in Kiri.
Sasuke shook his head at the suspicious looks Sakura was shooting Naruto. He could understand what the other boy was going through. If he had found himself on a battlefield across from a previously unknown relative, he himself wasn't entirely sure how he'd react. Especially if said relative was reluctant to fight him as Momochi Zabuza had appeared to have been with Naruto. He knew that wouldn't happen however, because the Uchiha had been careful not to leave any bastards for other villages to exploit.
"I'm sorry to say this, but you might have to watch that one." The bridge builder that they had been hired to protect said, gesturing to Naruto who was staring out the window. "The Uzumaki were highly family oriented and very skilled with seals. There were rumors of blood seals that prevented members of that clan from turning on their own before they scattered. If it comes down to a fight, he might be forced to protect his family despite the fact that he is your enemy."
Sakura's look of suspicion turned to horror.
"How do you know this?" he asked.
"Before the fall of Uzushiogakure, the Land of Waves was a territory of Land of Whirlpools and it had a satellite town that was linked to Uzushiogakure which was famed for its Seal Masters, and the Longevity of its inhabitants. The foremost clan amongst the inhabitants of Uzushiogakure being the Uzumaki clan. The Second and Third Shinobi wars destroyed the village and its protectorates and scattered the Shinobi of Uzu across the world. Following the Third war and the fall of Uzushiogakure, we were forced to build from the ground up without the Uzumaki to help us. When Uzushiogakure existed, it had very strong ties to Konoha as both the Uzumaki and Senju clans were distantly related, and the First Hokage's wife was an Uzumaki. In remembrance of this alliance, most of the Konoha ninja wear the symbol of Uzushio as part of their uniforms. It is because of this former alliance between Uzu and Konoha that we decided to turn to Konoha to help." Tazuna explained to the two Genin who acted like it was news to them when it shouldn't have been considering the fact that they had come from Konoha.
"The symbol of Uzushiogakure?" Sakura asked.
Tazuna simply pointed to the red spiral on the back of Naruto's jacket. The same red spiral that could be seen on the back of a standard issue Chunin or Jounin vest and on a great deal of standard and not-so-standard ninja attire.
Sasuke had thought that Naruto had worn that spiral on at least one article of his clothing at all times out of hubris, but it would seem that apparently wasn't the case. Naruto apparently had as much of a right to wear that symbol as he himself did the Uchiha fan. It marked who his family was and where they had come from.
A blood seal preventing members of a clan from turning on each-other was a very sensible idea; it was a pity his own clan hadn't employed one.
Sakura snuck over to Naruto's futon when she was certain he was asleep. She had to know whether or not her teammate was going to be a liability.
She heard Sasuke ask "What are you doing?" as he got up from his own futon, apparently haven been awoken by Sakura's furtive moments. Since Sasuke had lived on his own for the last four years since the tragic deaths of his family, any noise made around him while he was asleep tended to wake him up.
"I'm looking to see if he has any seals on him." Sakura replied, gesturing to the third Genin on Team 7. The one who was still asleep despite the fact that his teammates were moving around and talking.
"You do realize that you'll have to strip him to do that, and I seriously doubt Naruto would be able to sleep through that." Sasuke said.
Sakura rapidly returned to her futon despite the fact that she probably wouldn't be able to sleep. She blushed at Naruto's likely reaction to waking up to her removing his clothing, considering the crush he seemed to have on her.
"I'll check him in the morning while he's in the shower." Sasuke said, not making an issue out of what Sakura had almost done and what had almost ensued. Much to Sakura's relief.
The next morning, Sakura could barely sit still at the breakfast table as she waited for Sasuke. The loud yelling that was coming from the bathroom had indicated that he might have been successful. An eternity later, Sasuke arrived, followed by Naruto who was pulling on a shirt and glaring at Sasuke.
"I found two." Sasuke said to Sakura as he sat down at the table.
"Two what?" Kakashi-sensei - who had hobbled down just before her - asked.
"Seals. On Naruto." Sasuke replied.
"Odd. There should only be one, the large one on his abdomen that usually only appears when he's using Chakra. Where's the other one?" Kakashi replied.
Sasuke responded by pulling down the collar of Naruto's shirt. There was something there, on his back where his neck met his shoulder. It was nearly invisible, and if you weren't looking for it, you wouldn't have noticed it.
"...the hell?" Kakashi-sensei said as he examined the mark that almost didn't look like a Seal.
Edited 2-27-17