Fenrys was, in fact, returning back, but it wasn't until after the scuffle had finished by the time he picked up on the scent of blood.
It was the scent of blood - both Yukon's and another wolf's that he didn't recognize yet - that made his trot turn into more of a run. He'd never smelt Yukon's blood before, at least since he'd known Yukon. Which meant that either she'd prematurely taken her plan into action, or... something else had happened.
Fenrys didn't like the possibilities of "something else". Yet, as he approached with every swift paw, it was becoming clear "something else" was clearly what it was. For there was Yukon, lying on a rock ledge with said ledge seemingly coated in red.
Slowing down to a normal trot - no need for anyone to know he'd been running after smelling what he did - he quickly approached Yukon, looking at concern at her wounds, which were clearly wolf-made.
"Are you alright?" he asked before Yukon had the chance to say anything. Yukon was a powerful enoguh fighter from what he'd seen - for her to have this many wounds was... worrisome, to say the least. And that was another fact that he kept to himself.
"Who did this to you?" He took one look at Yukon before his ears flattened and a dark look flashed in his eyes. Padding in a circle around Yukon to figure out where exactly she was injured - usually he'd never show this level of concern, but Yukon was his one tie to outside this pack, and he wasn't sure how injured she might be - he tried recalling the other scent of blood, and who it might have been tied to, but at the moment he couldn't tell.
Finishing his circle around Yukon, he sat down on his haunches, ears still flat, and glowered at the "pack" they had joined while he re-asked the question. "Who did this?" While Fenrys didn't care about Yukon in an intimate way, at the very least he did care about whether Yukon lived or not. The pack that had invaded their territory held almost zero care for him, and although he didn't care about Yukon intimately he did care that someone had done this to her. In a protective way - they had looked out for each other for quite some time now, and this was no different.
Whoever had done this had probably just asked for a death sentence. Maybe not now, but eventually.