That wasn't a jerky reply at all!
Vs mark 1 eyeballs and ears, assuming you can slow re-entry so you're not a flaming meteor. . .You could sneak into the earth, early or pre WWII and never be discovered. . .and likely not until well into the cold war you could just come in over the congo, amazon, sahara, either pole or mid ocean in a lot of places and either not be detected, or be detected and dismissed.
As radar coverage increased, you'd need to be more careful and pick holes. . .if you can sense their sensors further out than they could detect you you'd likely be able to find holes to settle into. . .even on modern earth there are places not watched that carefully.
Once you get a global network of geosynchronous orbiting platforms watching for fast moving objects on radar (like missiles) or infrared flashes (like missile launches) you'd need to be more careful. (If they mostly look down, you might be able to sneak up on one, co opt or slightly damage it, then re-enter the hole that created).
Speculating from here on future tech
The next step would be orbital platforms watching out. . .
the next step from that would be solar orbiting platforms watching the whole solar ecliptic.
The next step from there would be sensors above and below the ecliptic watching down into it and out from it.
From there a sphere of sensors watching the whole area inside the Oort.
Then a whole sphere watching from outside the Oort.
Ground based sensors, would be limited by horizons, and except on perfect cue ball planets by masking be terrain features. . .and unless you build mid ocean platforms or keep up a rotating patrol of scanner ships, total coverage is hard to tighten over large oceans. . .much easier to launch orbital platforms.
From there, it's quality of sensors vs quality of stealth, how well integrated the sensors are, and how smart/skilled the people or AIs watching them are.
