I am likely to face this problem very soon. When we opened our front door this morning we found 2 pints of milk on the doorstep in glass bottles. Certainly not ours, we use very little milk.
Our various neighbours are at work, so I have popped postcards in their doors to say, if it is yours come and claim it, but so far no one has.
I have rung the dairy whose milk it is, and they have given me the name of the local supplier, but they have packed up for the day, so if no one claims the milk this evening, I will ring them first thing tomorrow.
Meanwhile the milk is in my fridge.