More likely a workout heartbeat monitor. If a bikini, it's under the breasts, so it should have an upper strap around the back of the neck, but we see no tanline there.

Could also just be fake...

Edit: according to The Tineye, the tanline is legit, so I stick with the heartbeat monitor, or similar.

More likely a workout heartbeat monitor. If a bikini, it's under the breasts, so it should have an upper strap around the back of the neck, but we see no tanline there.

Could also just be fake...