If I was an intelligence agency, I would do the trivially obvious thing and only use "unmarked cars" when I didn't care about being spotted, and an actual nondescript vehicle the rest of the time.

What's the difference between an "unmarked car" and a "nondescript vehicle" ?

You think the CIA would do their clandestine work on cars labeled "CIA" ?

> What's the difference between an "unmarked car" and a "nondescript vehicle" ?

Unmarked police cars often have multiple radio antennae, flexible lights, and even government plates, they simply lack explicit police markings and light bars.

Surely an unmarked van owned by Apple would have none of that?

The point is that the "unmarked" vehicle sticks out as unusual even without having "Apple" or "Police" emblazoned on its side.