ADS-B Exchange and others of its ilk only know of aircraft (and balloons for that matter) that squawk or transmit ADS-B signals. The military can make aircraft stealthy to these apps.
For example, globe.adsbexchange.com depicts air refuelers apparently very reliably. Sometimes 40-80 of them are up at once. But, as to what they are refueling... fighters... we rarely see them. From ADS-B POV, the refueler goes up, goes in circles by itself, and then lands.
Yes, quite true.
But there are a lot of people whose hobby is to watch airfields and such, and takeoffs without showing up on ADS-B are likely to be noticed and commented upon, especially in the context of a national news story. As are are diversions of other traffic around a spot where they’re doing something- see a bunch of commercial or general aviation traffic avoiding a certain spot, or just see an empty area of airspace that normally isn’t, and you know *something* is up.
For something like this (a shoot-down) with falling debris, you can probably count on a temporary closure of airspace or a NOTAM. There’s probably going to be some indication that something abnormal is going on.
That said, you are correct- if they chose to do it in the middle of the night (less commercial aviation and very little general aviation) in the middle of nowhere, they might be able to get it by unnoticed as something like a “routine training exercise.”
You’ve actually got me wondering- would a better way to cover up a covert shootdown be to just do it in the middle of the night and leave the intercepting aircrafts’ transponders on, while accomplishing the deed with as little deviation from a normal flight profile (and little or no airspace closure) as possible..?
(If some civilian notices: “Why yes, we did have an exercise at 0200 last night. I can’t give you the details, but it was routine in nature and all affected parties were notified in advance. Please forward any other questions to Lt. So-and-So of our public relations department...)