Transponder settings had to have been compromised. The four digit identifier that pings on the radar screen identifies the aircraft or drone or missile. If the base radar sees valid four digit codes as radar returns as friendly radar contacts, no alarms would happen.
Remember the drone that crashed in Iran and Obama refused efforts to retrieve it? It was likely reverse engineered and this was a trial run to see if their assumptions would work.
Maybe the four digit code was set as 1,2,3,4,?