It can be difficult when kid's go away to college and come home for the summer. Maybe they have a summer job, they've gotten used to some freedom and making their own choices. They may think they don't need to tell you where they're going or what they're doing anymore. Explain to them that it's just common courtesy for people living under the same roof to communicate with each other about their schedules. Also let them know that you still care about them, so it's natural for you to have concerns.
Do you have adult children at home? Do they still do their chores?