WEST COVINA, Calif. — After a boozy Saturday night, Sarah Maguire awoke the next morning to find that her iPhone was gone. Her roommate’s phone was gone, too. Were they at the bar, she wondered, or in the cab?

Using the Find My iPhone app on her computer, she found that someone had taken the phones to a home in this Los Angeles exurb, 30 miles east of her West Hollywood apartment.

So Ms. Maguire, a slight, 26-year-old yoga instructor, did what a growing number of phone theft victims have done: She went to confront the thieves — and, to her surprise, got the phones back.

“When I told my mom what I did, she thought I was crazy,” Ms. Maguire said.

With smartphone theft rampant, apps like Find My iPhone offer a new option for those desperate to recover their devices, allowing victims like Ms. Maguire to act when the police will not. But the emergence of this kind of do-it-yourself justice — an unintended result of the proliferation of GPS tracking apps — has stirred worries among law enforcement officials that people are putting themselves in danger, taking disproportionate risks for the sake of an easily replaced item.