On a Sunday night in September, Ashley Estrada was at a friend’s home in Los Angeles when she received a strange notification on her iPhone: “AirTag Detected Near You.”

An AirTag is a 1.26-inch disc with location-tracking capabilities that Apple started selling earlier this year as a way “to keep track of your stuff.” Ms. Estrada, 24, didn’t own one, nor did the friends she was with. The notification on her phone said the AirTag had first been spotted with her four hours earlier. A map of the AirTag’s history showed the zigzag path Ms. Estrada had driven across the city while running errands.

“I felt so violated,” she said. “I just felt like, who’s tracking me? What was their intent with me? It was scary.”

posted on TikTok, Reddit and Twitter about finding AirTags on their cars and in their belongings. There is growing concern that the devices may be abetting a new form of stalking, which privacy groups predicted could happen when Apple introduced the devices in April.

warned its community of the tracking potential of the devices after an AirTag was found on a car bumper. Apple complied with a subpoena for information about the AirTag in the case, which may lead to charges, West Seneca police said.

And in Canada, a local police department said that it had investigated five incidents of thieves placing AirTags on “high-end vehicles so they can later locate and steal them.”

Researchers now believe AirTags, which are equipped with Bluetooth technology, could be revealing a more widespread problem of tech-enabled tracking. They emit a digital signal that can be detected by devices running Apple’s mobile operating system. Those devices then report where an AirTag has last been seen. Unlike similar tracking products from competitors such as Tile, Apple added features to prevent abuse, including notifications like the one Ms. Estrada received and automatic beeping. (Tile plans to release a feature to prevent the tracking of people next year, a spokeswoman for that company said.)

stalkerware.

“Apple automatically turned every iOS device into part of the network that AirTags use to report the location of an AirTag,” Ms. Galperin said. “The network that Apple has access to is larger and more powerful than that used by the other trackers. It’s more powerful for tracking and more dangerous for stalking.”

Apple does not disclose sales figures, but the tiny $29 AirTags have proved popular, selling out consistently since their unveiling.

An Apple spokesman, Alex Kirschner, said in a statement that the company takes customer safety “very seriously” and is “committed to AirTag’s privacy and security.” He said the small devices have features that inform users if an unknown AirTag might be with them and that deter bad actors from using an AirTag for nefarious purposes.

“If users ever feel their safety is at risk, they are encouraged to contact local law enforcement who can work with Apple to provide any available information about the unknown AirTag,” Mr. Kirschner said.

Police could ask Apple to provide information about the owner of the AirTag, potentially identifying the culprit. But some of the people who spoke with The Times were unable to find the associated AirTags they were notified of and said the police do not always take reports of the notifications on their phones seriously.

After a Friday night out with her boyfriend this month, Erika Torres, a graduate music student in New Orleans, was notified by her iPhone that an “unknown accessory” had been detected near her over a two-hour period, moving with her from a bar to her home.