Hmm... can you talk to them and find out where they got the dog from?
If it's from a reputable breeder (will be between 7-9 weeks old), then the breeder will have something to say.
If it's from a rescue centre I promise the rescue centre will have something to say.
Your concern is right... If you can't fine out where the dog is from, then report it to the dog warden. They can take action.
A puppy (like a baby) ought not to be left alone....
You are doing the right thing by questioning this.