I have a feeling Luke was thinking maybe Mathew survived the shot and he could probably try saving him sense Kenny told him to leave. Maybe Kenny didn't leave and Luke went off by himself. Luke felt bad about what Nick did so he was thinking he could see if Mathew was still alive somehow. I'm guessing there will be a twist where Luke ends up saving Mathew. Mathew will be somewhere on the share wounded but not dead. It's possible someone can live with a neck shot. They will just be in pain. There was this guy in my neighborhood who was shot in the neck and we took him to the hospital.
Now remember the scene where Clem is climbing the ladder and see's lights? Those lights she saw was Carver and his group. It's possible Carver made his group search around the bridge for anything. They could have spotted Mathew and took him to their camp. Carver and his group questioned Mathew if he seen anyone lately. Mathew mentioned the guy who shot him and Luke and a little girl. After that Carver wanted to know the location of where they went. Mathew told them the Ski Lodge. Carver then wanted Bonnie to go scout for him and check the Ski Lodge out. How else do you think they would have knew the location? Without Mathew's help Carver's group wouldn't even know about the Ski Lodge, so my guess is Mathew's alive and held as hostage in Carver's community. Mathew will be upset Walter is dead and want Revenge.
I know this theory has to be good because it sound plausible.
The lights were Carver and his group's flashlights
They scouted around the bridge to look for clues
Mathew was alive and told them about Nick, Luke, and Clem, and the location of the Ski Lodge
Carver sent Bonnie out to scout the Lodge
Bonnie went back to Carver and led them back to the lodge
Mathew is at the Carver community in the mall as shown in episode 3
Luke went to find Mathew, but Mathew is already at Carver's community
Luke will go back to Carver's community to save the group sense he already knows the location
Oh and Mathew could have told Carver a girl named Clem mentioned a guy named Carver, and that's when Carver wanted the location. They either killed Mathew after he gave them information, or they took him back to the community before going to the Ski lodge
This is my theory please feel free to discuss :)