They can mask ADSB.
They can turn off their transponders.
What you can see, anyone can see.
"Kike"
Squawk number is set by the flight crew based on:
A request from the local air traffic control to set tranpsonder so ATC can better track and communicate.
Or
To communicate status or problems should standard radio not work, not be needed.
1200 - Uncontrolled airspace
7500 - Hijacked
7600 - Communications down
7700 - Mechanical or medical emergency