furtive vs under the radar

Of a thing: done with evasive or guilty secrecy. 

Of a thing: that has been acquired by theft; stolen; also (generally) taken stealthily. 

Of a person or an animal: sly, stealthy. 

Of a person, etc.: inclined to steal; pilfering, thieving. 

Without attracting notice; in an undetected or secretive manner.