a escondidas

adv. on the quiet

= by stealth, stealthily, furtively, on the quiet
Ex: He is a systematic 'sweater' who sucks wealth from toiling crowds by cunning and by stealth. Ex: It is contended that adoption policies have encouraged nondisclosure of information by gay men & lesbians, & surrogacy arrangements are often handled stealthily. Ex: Seditious books continued to appear, nevertheless, both from secret presses in England moving furtively from hideout to hideout. Ex: Lee is normally a very quiet member of the crew, although we suspect that on the quiet he is a bit of a lad.