Your car is tracking you. Abusive partners may be, too.

After almost 10 years of marriage, Christine Dowdall wanted out. Her husband was no longer the charming man she had fallen in love with. He had become narcissistic, abusive and unfaithful, she said. After one of their fights turned violent in September 2022, Dowdall, a real estate agent, fled their home in Covington, Louisiana, driving her Mercedes-Benz C300 sedan to her daughter's house near Shreveport, five hours away. She filed a domestic abuse report with police two days later.

Her husband, a Drug Enforcement Administration agent, didn't want to let her go. He called her repeatedly, she said, first pleading with her to return, and then threatening her. She stopped responding to him, she said, even though he texted and called her hundreds of times.

Dowdall, 59, started occasionally seeing a strange new message on the display in her Mercedes, about a location-based...