A drone powered by a radioisotope thermoelectric generator could stay in the air indefinitely and have a range of thousands of miles. We have used RTGs to power satellites and small spacecraft for over 50 years.
What propulsion system were these decaying element reactors powering? There are no visible rotors in any of the videos. Rotors have reasonably pronounced visual signatures when observed in maritime climates using night vision optics.
https://www.youtube.com/watch?v=ioFv-3tND7A
I've flown off helo carriers more than a 100x. Just about every time, the visual cue seen above is visible if you're wearing NVGs. Sometimes, it's even visible if you're not wearing NVGs. It has something to do with static electricity.
If they're not being powered by rotors - and it evident that they're not - how are they staying aloft?