This is par for the course, Davida, and I can assure you that grandsons are no easier at that age. All right, boys don't spend oodles of time in the bathroom making up and doing their hair, but they eat you out of house and home.
Yes, you can confidently expect a nice adult to emerge after the teenage years are over and done with.
You don't say how old they are exactly, or how long they are staying for, but in your place, as you have generously taken them on trips and probably paid for these, I would sugget you mention today or tomorrow that you haven't planned anything for next week, as you imagined they would want to do their own thing for some of the time.
Ask them to tell you where they intend to go before they set out, and when they will be back.
Fix a definite time for their return and warn them that if they are not back at that time, you expect them to have phoned you to say that they are delayed. If they do not either phone or come back at the stated time, you will have no compunction about phoning the local police and hospitals.
If they are over 18, which I assume, as you say they technically are adults, you cannot demand this, but tell them frankly that you will worry if you have not heard from them and they don't come back as agreed.
If they are under 18, you can demand this, as you are legally responsible to their parents for their safety