In the age of cell phones, I think I'd start by calling them... and hope they picked up!  If not, then I would be on the phone to the police to help me find them before they hurt themselves or someone else. The trouble they get in with the police won't be 1/2 as bad as the trouble they would be in with me.

When my kids were young, I always told them that if they found themselves in a situation where they had been drinking or others they were with had been and they didn't have a way home either to stay put for the night or call me.  I promised I wouldn't yell and fuss at them at that time, but would come and get them and take their friends home too if need be.  I might not be too friendly the next day, but there would be a next day...I wouldn't be identifying their bodies at the morgue.

I also told them I would turn them in to the authorities if I ever knew they were out drinking and driving.  It is that way even now that they are grown and out of the house.  I have no sympathy for a drunk driver...there are no any excuses.