Saying a coyote killed a small-breed dog in New Canaan last week, authorities are urging residents to be careful about leaving their pets outside unattended. The remains of a terrier let out back of a Beacon Hill Lane home on the night of March 6 were found the following morning after the animal did not return home with another dog that had been let out, according to Officer Allyson Halm, head of the New Canaan Police Department’s Animal Control section. Clearly defined canine tracks led to the deceased dog’s body in the woods, she said. Halm said that coyotes are a fact of life in this area and that a lifestyle change on the part of residents is required to co-exist with them. “Our guard cannot be down,” Halm said.