"You're not sure?" Papyrus asks, frowning and considering their options. He doesn't have any idea what

Unfortunately, blue magic isn't an efficient way to detect invisible people. Searching the entire sphere of the world around oneself for signs of a soul to ping would take a while, and people can move in the meantime. If it could be efficient, ghosts wouldn't be so spooky...

"Well... what if we keep moving, and whoever you're feeling doesn't follow?" And, really, Reira was just injured in that Prism. Maybe she's... perceiving things that aren't there? "We won't know more than we know now, but we'll be closer to friends."