They are more active at night. Also, slugs are a little more active during the cooler hours of the day and during the summer months. They also like moist conditions (think about what a slug likes to eat).
Slugs are found in nearly all of the United States, Canada, and throughout Europe. While they are not considered invasive pests in many areas, slugs do spread disease, such as Botrytis cinerea, which is known as gray mold. Gray mold causes leaves to turn brown and then dry up.