More than likely that conjectured capability is the yesteryear Predator.

Seems tech may have received some upgrades:

https://www.technologyreview.com/2019/06/27/238884/the-penta...

There was also a hot-air balloon program that would persist at stationary at high altitude and track all moving targets over a metropolitan area for up to a month without maintenance.

The news article which said the project was called "Basilisk Stare", seems to have vanished now though from all search results. Edit: Maybe I'm misremembering and its actually Gorgon Stare.

https://en.wikipedia.org/wiki/Gorgon_Stare

>There was also a hot-air balloon program that would persist at stationary at high altitude and track all moving targets over a metropolitan area for up to a month without maintenance.

They could do this with a single (bajillion dollar) military radar slung from the bottom of a plane over Iraq 15yr ago.

The current tech is probably cheaper and better.

DHS has these and is already using them. There is an aerostat that flies in South Padre whenever SpaceX is launching starship doing exactly what you suggest.