As a side note, if the theory of the pico balloon is correct, then the US airforce is capable of radar-identify a 32" diameter mylar balloon and shoot it with a missile from a fighter jet, which seems to me like being exceptionally accurate.
That said, they did miss on the first attempt with the Lake Huron balloon, so I guess it's harder then I'd have thought.
It either needs a hot part for an IR seeker to lock on to, or be radar reflective enough that a high power radar beam can reflect off and be picked up by the missile's antenna and pass a certain threshold of reflected power. There's not much heat in the payloads, and balloons probably aren't the best radar reflectors.
Even gun lead calculations require a good radar return.
I've always predicted we would eventually end up with missiles that guide on visible light, but that's terrifying if it isn't nearly perfect.