Someone who had been following them.

Natsu sniffed the air, smelling someone familiar.

"Okay, Little Pet, stay here." Said Natsu before darting towards the source of the sound.

The voyeur had no chance to get away. They couldn't match Natsu's speed, and his keen nose prevented them from being able to hide from him. He quickly reached and subsequently restrained a blue haired woman. "Hey, Juvia! It's been a while!." He said. "Have you been watching for long?"

The woman stood immobile for a second with a look of shock, just staring at the naked man grabbing her wrists, but she couldn't take his inquisitive look and found herself nodding. "J-J-Juvia saw e-everything" She stammered, blushing as she spoke. "Juvia saw you doing t-t-those indecent things with Grey-Sama."

"And I assume you are not going to just forget it and pretend like nothing happened, are you?"

"Juvia will never forgive you! Juvia should have been more careful, Juvia let her guard down because Juvia never considered you a romantic rival, and you took advantage of it to steal him from Juvia! If only Juvia had known Grey-Sama better, Juvia would have known you were a threat..."

"As I feared. I guess I'll need to do something about it, then..."

EDIT: I can't believe after a fucking week of planning this chapter I forgot how Juvia speaks of herself in third person. I swear to the Erokami I had it in mind when I first envisioned this chapter, it just slipped as I got horny, I guess. Thankfully, I realized while writing the next one, so I can fix it quickly.