It was Clint and Natasha who ratted him out. It was always Clint and Natasha.
More specifically, it was Clint who found the video on YouTube and Natasha who decided to show it to Tony.
Peter was certain Aunt Natasha had timed it. She knew which days he worked, which days Gwen worked, and which days they were both off and would spend the afternoon together.
Specifically, that was a day Gwen was working, so Peter came home after school, and Aunt Natasha being Aunt Natasha, she had it timed down to the minute when Peter would be home.
Which is how it came to be that Peter walked in right as the video ended. “Hey, dad!” he called into the living room, setting his bag and skateboard on the kitchen table – it was Uncle Steve who always yelled at him for that, and the Cap was off on a mission at that particular time. He poked his head into the room, smiling. “I’m home.”
From the look his father was giving him, Peter knew immediately that something was up.
“Something you want to tell me, Peter?” Tony asked in that strangely calm tone he always got when Peter was in for a lecture about his powers.
“Um, no?” He knew it was pointless, but what the heck? It really couldn’t get any worse, could it?
His eyes skipped to Nat, what was standing beside Tony with her hands clasped behind her back, head bowed slightly to hide the small smirk of amusement.
Tony nodded. “Really? Well-“
And Loki breezed into the room. “Hello, Peter,” he smiled, then he realized who else was in the room, too. “Ah, Natasha! What a surprise.”
“Hi, Loki,” she nodded. It didn’t seem like she’d ever become completely comfortable with Loki being around as much as he was, but she never said anything about it, and at least she could be in the same room with him and carry on a conversation without needing her hand on her gun. That’d been a particularly awkward phase for all involved and all had been glad when it passed, including Natasha.
“And to what do we owe the pleasure of your company?”
“Babe, you might want to see this,” Tony said, reaching over to pull him to his side and hitting replay, though his eyes never left Peter.
Even from where he was standing, Peter could hear exactly what was happening on the video. Some guys were getting the shit kicked out of them. Presumably by him.
Loki’s expression remained mostly blank as he watched, but Peter could see the amusement in his eyes. The video was short, less than a minute, so it wasn’t long before Loki was looking to him, a grin spreading across his face. “Nicely done.”
Tony choked. “Nicely done? You saw what he did! He could have been hurt!”
Loki ignored him, still looking at Peter. “We can tell which one of us you’re taking after, can’t we?”
“How are you happy about this?” Tony seemed a bit hysterical at that point. “What if he’d gotten hurt? Peter, I thought we’d agreed that you weren’t going to use your powers out in public.”
“He handled himself well, Tony,” Natasha piped in.
“You see? He did well!” Loki rolled his eyes and sat down across Tony’s lap. “Let the boy have some freedom, Tony. If it will make you feel better…” He turned to Natasha. “Would you be willing to give him some training? Teach him some techniques?”
Natasha smiled at them, though it was the smile of someone who was going to enjoy something a bit too much – in this case, whooping Peter’s ass into the mats at the gym. “Of course, anything for my darling nephew.” There was nothing darling about the look she was giving Peter. That look was completely made of the anticipation of fun and pain.
Tony’s gaze flicked wildly between them all as Natasha led Peter from the room, telling him exactly how much fun they were going to have, but he couldn’t quite find proper words.
“Relax, Tony,” Loki said soothingly. “We have to loosen the reigns sometime. We can’t stop him, we might as well help him.”
Tony grumbled something that Loki couldn’t quite make out, but Loki knew he’d won. He pressed his lips to Tony’s cheek and stood, throwing him a devious look. “Finish what you’re doing there and then come find me.”
Tony’s attention was immediately shifted. He pushed his chair back, smirking. “I’m done now.”
Loki grinned as he pulled Tony along with him. “Good.”