Placeholder. Headphones are quietly becoming health-monitoring wearables — Beats' newest earbuds include a built-in blood/heart-rate sensor, and Apple is reportedly developing AirPods with embedded cameras, both moves that push headphones toward the same wearables/AR territory currently occupied by smartwatches.
What we've found so far: Beats Powerbeats Pro 2 ships with an embedded blood/heart-rate sensor targeting gym-goers; per Bloomberg's Mark Gurman, next-generation AirPods are reportedly in development with embedded (likely low-res/infrared) cameras.
Open research: Track official confirmation/launch of the camera-equipped AirPods once announced; interview an analyst on whether this genuinely threatens the standalone smartwatch category.