It's true that you don't want to encourage secrecy, and you want them to trust you enough that they call you. At the same time, you don't want them drinking too much!
You're not in legal trouble, OP, since you don't have "evidence" they were drinking. No bottles, no vomit, no nothing. And police probably won't bother themselves for teens who drink quietly, don't wake up the neighbors, don't drive drunk and call their parent instead...
But I know how you feel. It's not *right*. At the same, good job parenting the child who called you