Today the US Patent & Trademark Office published a patent application from Apple that relates to a next-generation of AirPods Sensor System wherein the housing and tips of AirPods could include a number of active and reference electrodes configured to measure biosignals and electrical activity of a user's brain.
Definitely would not be used to monitor user data and then used for targetted advertisements. \s
On the side note, this could be pretty slick if open source community is able to use them for all kinds of purposes.