Future incarnations of the Apple Pencil could have a built-in antenna. Apple has filed for a patent (number 20150363013) for an “antenna for a computer stylus.”
I realize that Apple calls its device a pencil, not a stylus, but you get the idea. The stylus/pencil as described in the patent would have an elongated body with a metal tube that served as an antenna ground for an antenna.
Why would an Apple Pencil need an antenna? According to the patent filing, such a device could run software. Said software could process input from buttons, sensors, and other input components. The software may also be used to provide output to a user. The antenna would be used to support interactions with external equipment such as an iPad.