Martin Ringlein never shared his Netflix password with anyone. So he was shocked to find out that strangers had been using his account to watch movies and TV shows.

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One of them was the person who bought his Tesla about a year ago, Mr. Ringlein said. Others were guests at an Airbnb in Asheville, N.C., where he had stayed weeks earlier. Mr. Ringlein had watched Netflix in his car and at the Airbnb, but hadn’t signed out—and the strangers apparently took advantage of that goof to mooch off his account.