Comment by Animats

Here's a 10 year old study from Norway's navy.[1] It's a marginal technology for navies. It's more useful for defending urban areas from air strikes, because urban areas have lots of emitters, too many to attack.

There's a lot of current interest in what Iran is using for air defense. Search "Iran air defense passive radar". Some people speculate passive infrared. Some speculate passive radar. Nobody who's posting really knows, of course.

Ukraine uses many passive systems, including audio. Sort of like Shot Spotter, but for drones.

[1] https://apps.dtic.mil/sti/pdfs/ADA470685.pdf