Guessing... something to do with listening or viewing, meaning; either drones, satellites, or fixed wing aircraft are getting a fix on the HVTs, due to advanced technology. On the other hand, it could be something else entirely... maybe a heat signature that, come to find out, is as individualized as DNA? Or a voice print?
Thankfully, Woodward didn't run his mouth too much and reveal the technology.
Leave it to the press... we'll know sooner rather than later.