It wasn't all that strange that Marco didn't feel anything, really. He wasn't a creature of the Underworld. He didn't have that innate sense of all things dead and dying.
"I don't know," he said. "It started a few days ago, around the time the creature was destroyed. I don't know what it is but there's just this...disturbance." He couldn't put a finger on it, except that it wasn't right.
"Something's out of balance and I need to find it and help it get back."
Otherwise, this was going to nag at him for a long time.