If you have teenagers, chances are good that they babysit, and it is very common for families to have rules about babysitting. For example, while babysitting teens should:
not have friends over or talk to friends on the phone;
not babysit until after 9 PM on school nights;
have a contact phone number for the parents during the evening;
have a permission to authorize medical treatment;
know the insurance information for the family; and
agree on fees before accepting the job.
It is also common for the family s/he is working for to bring her home, but have you ever specifically said to your teen that if the parents s/he was babysitting for appear to have been drinking, s/he is to call you for a ride home instead?
Given that most people seem to understand "it is never OK to drink and drive," I cannot say this would have occurred to me, but I think it is extremely important and well worth a conversation.
Teens sometimes tend to trust adults, and your teen may not stop and think about getting into the car with a family friend who has been drinking the same way you expect him or her to with a peer, so please, talk about it before s/he makes a poor decision. Let your teen know that you would rather get woken up to drive a child home then for him or her to get a ride with someone who had been drinking.