under the radar

:  in or into a position below or beneath something

:  below or short of some quantity, level, or limit

:  in or into a condition of subjection, subordination, or unconsciousness

:  below or beneath so as to be overhung, surmounted, covered, protected, or concealed by

:  subject to the authority, control, guidance, or instruction of

:  receiving or undergoing the action or effect of

:  lower in rank or authority :  subordinate

:  a device or system consisting usually of a synchronized radio transmitter and receiver that emits radio waves and processes their reflections for display and is used especially for detecting and locating objects (such as aircraft) or surface features (as of a planet)

:  range of notice