Yup, long stare SAR hooked up to a DVR was a thing over 10 years ago - it could cover a lot of ground (like a large Iraqi city) with a resolution that would easily detect this drone.
Not - one drone. A drone in the air 24/7 every 5km for a border ~2000 km long.
But even if you need a drone every 5km (you don't). That's all of 400 drones, 800 if you want to have pairs doing charge-work duty. For an 2000km border guarding operation, that's really not much. An order of magnitude or two cheaper than the fence they would guard.
That's probably worse than those drones watching from above, seeing as you'd need actual infrastructure at every interval. And, that'd be fixed infrastructure, with fixed blind spots.