Using the Find My iPhone app on her computer, she found that someone had taken the phones to a home in West Covina, 50 kilometres east of her West Hollywood apartment.
So 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," Maguire said.
"This is a new phenomenon - it's not simply running after the person to grab the phone," said George Gascon, the San Francisco district attorney and a former police chief. "It opens up the opportunity for people to take the law into their own hands, and they can get themselves into really deep water if they go to a location where they shouldn't go."
"Some have been successful," Gascon said. "Others have gotten hurt."
More than 3 million smartphones were stolen across the United States last year, according to a survey by Consumer Reports.
Victims are often desperate to recover their stolen phones, which, as home to their texts, photos and friends' phone numbers, can feel less like devices than like extensions of their hands.
And although pursuing a thief can occasionally end in triumph, it can also lead to violence, particularly because some people arm themselves - hammers are popular - while hunting for their stolen phones.
A New Jersey man ended up in custody himself after he used GPS technology to track his lost iPhone and attacked the wrong man, mistaking him for the thief.
Commander Andrew Smith, a spokesman for the Los Angeles Police Department, called the trend "a big concern".
"It's just a phone - it's not worth losing your life over," he said. "Let police officers take care of it. We have backup, guns, radio, jackets - all that stuff civilians don't have."
Still, although police departments have devoted more resources to combating smartphone theft, most cannot chase every stolen device right away, especially if the phone was left idly on a bar rather than seized in an armed robbery.
Despite the obvious risks, the lost phone's location - blinking on a GPS app - is a siren song many find too alluring to ignore.
After Nadav Nirenberg lost his iPhone on New Year's Eve in 2012, he realised someone was sending messages from his OkCupid account. He lured the thief to his Brooklyn apartment building by posing as a woman and flirting with him on the dating service.
When the thief arrived with a bottle of wine, expecting to meet "Jennifer", Nirenberg went up behind him, hammer at his side. He slapped a $20 bill on the thief, to mollify him and compensate him for his time and wine, and demanded the phone. The thief handed it over and slunk away.
"I was trying to avoid conflict," Nirenberg said. But he added that, if robbed again, he would go to the police.
Police chiefs have advocated another solution that they say could end smartphone thefts altogether: a mandatory "kill switch" that would render stolen phones inoperable and therefore unattractive to thieves.
"This would all be moot if we had an industrywide kill switch," Gascon said.
After years of pressure, phone makers, including Apple, have begun offering this feature. But a bill that would require a kill switch on all smartphones sold in California has stalled in the state legislature amid opposition from the telecommunications industry.
When Maguire and her roommate called the Los Angeles police, she said, they were told they could go to West Covina themselves and call 911 if they felt threatened.
Maguire debated for hours before deciding to go after her phone. The house where her phone had been taken was on a quiet residential street, with a well-tended yard and palm trees out front. Inside, she could see children running through the hall - a sign, she hoped, that she was dealing with opportunists, not career criminals.
She knocked on the door. It swung open, revealing a large man, about 30 years old. "I think you have my phone," Maguire said haltingly, as she later recounted the conversation. The man denied this. But she pressed him, insisting that GPS had led her to that address.
The man ducked back inside. The blinds in the living room, which had been open when she arrived, slowly closed.
Finally, he returned with one phone, then - after more negotiation - with the second. Unsure if the danger was over, Maguire sprinted to her car.
Still, when she was asked by text message if she would pursue a future pickpocket, she typed an unequivocal reply on her recovered phone: "Yes, def."
New York Times