Whittaker's food safety manager Claire McKay said the complaint was lodged via email and the company immediately issued an apology and requested the chocolate be sent back to its Porirua factory to be investigated.
Whittaker's hired an external electronic expert Keith Bowen who reported it was a sophisticated tracking device which had been placed in the bar after it left the factory.
A report obtained by the Herald on Sunday said the device - 10mm in diameter, 4mm thick and containing a circuit and aerial - was not planted in the slab during the manufacturing process as it would have been found by quality control systems.
It also said the molten chocolate would have leaked into all the cavities and ruined the device, indicating it had been put into the slab once solid.
"Extraordinary as it may seem, the most likely way this device got into the chocolate slab seems to be by deliberate placement," said Bowen.
McKay said the Whittaker's factory had two metal detectors for each product line, which were tested every two hours, as well as x-ray machines which would have picked up the device.
Paragon Investigations director Ron McQuilter said the tracking device would have been highly technical and expensive because devices that tiny were difficult to find.
"Something that small isn't normal," said McQuilter. "I'm imagining it would be very high-tech which means it will be expensive."
He suggested it could have been taken from animal researchers or made at home then placed in the chocolate to trace the woman's movements.
"It sounds like a lunatic friend with personal issues going on as opposed to someone at Whittaker's doing it," said McQuilter.
"That would have taken a while to do, a determined person has done it. He has had to open the wrapper and glue it back again.
"You see this kind of thing on the movies but you try opening something and put it back together again, it's not easy."
He said anyone could buy a larger tracking device on the internet for about $70 which was "scary".
"Technology has moved with the ability to plant a small tracker, which is effectively a small phone which you can dial. In the old days you put the device in and it had a battery on it and the battery would run out."
Security Risk Management managing director David Horsburgh said it was unlikely the person wanted to track a chocolate bar.
"What needs to be analysed is what was sent with the package," Horsburgh said.
Whittaker's did not lay a complaint with police.
Condom surprise in quarter pack
A KFC customer claimed to have found a condom in their quarter pack meal from Hamilton's Frankton store in February last year, Restaurant Brands spokeswoman Jo Bell said.
She said it was not known how the condom got in the meal or if it even came from the franchise. She said the incident was taken very seriously, with formal investigations undertaken internally and with external authorities, including the Ministry of Agriculture and Forestry.
"The store manager was informed several months later by the Waikato District Health Board officer assigned to the case that 'it was a prank by a flatmate of the person concerned'," Bell said.
As a result, KFC has installed cameras in all of its stores' kitchens, serving and customer areas.
Food Safety New Zealand consultant Suresh Din encouraged customers to inform authorities about foreign objects in their food, poisoning or a lack of hygiene.
