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.