I had a work-study job in college at our small local airport working for the FAA. I mostly clipped new FAA communications into 3-ring binders and tore off expired ticker tape weather reports for a couple of hours a day.
My boss was old school government employee, straight and square. One day we were talking and he handed me a FAA business card with a UFO reporting 800 phone number. Odd I thought, and put it in my wallet.
Years later I lost my wallet in a phone booth (am old). I got a call from someone who found my wallet and wanted to return it. He came over with the wallet which had everything still in it, including cash. The only thing missing was the UFO card. I've never seen another one.
No, like a lookout not reporting every seagull he sees. Not everything that reflects radio waves is an enemy aircraft, so you're always going to be culling signatures that are too small or too slow. If you build a radar station to watch for bombers and ICBMs crossing the arctic then a balloon flying at or below windspeed isn't what you're looking for.