Two Apple Watches sit side by side.

Updated September 2017: The Red Sox may not have used an Apple Watch to steal signs.

Nick Cafardo, a baseball writer for The Boston Globe, cites a major league baseball source who says the Boston Red Sox didn't use an Apple Watch to steal signs, but instead used a FitBit device. Some FitBit products feature app and messaging notifications along with their standard activity tracking capabilities.

From The New York Times:

The Yankees, who had long been suspicious of the Red Sox stealing catchers' signs in Fenway Park, contended [a] video showed a member of the Red Sox training staff looking at his Apple Watch in the dugout and then relaying a message to players, who may have then been able to use the information to know the type of pitch that was going to be thrown, according to the people familiar with the case. Baseball investigators corroborated the Yankees' claims based on video the commissioner's office uses for instant replay and broadcasts, the people said. The commissioner's office then confronted the Red Sox, who admitted that their trainers had received signals from video replay personnel and then relayed that information to some players — an operation that had been in place for at least several weeks.

Certain electronic devices are permitted for use, but they aren't to be used for communication. The New York Times says it's still unclear whether The Red Sox will face penalties for using the electronic devices in this way, though the event could impact the use of communication devices on and around the field.