[Sirius Lee]

But before you break out your book of conspiracy theories, we should be clear that researchers can't, under any circumstances, use these tiny devices to track you; they don't even use the type of radio frequency (RF) communications that would be required to do so.

Don't "use", but do they "have" an RF signature?

[Rurudyne]

Also, the RF used can be changed. And expect capabilities to greatly increase over time.

[bigbob]

NO. No RF involved anywhere, just ultrasound which requires a transducer that is in contact with the body.