Not seen or observed:
— Managed to slip away unobserved.

synoniem: unseen.

Roget 460: neglecting etc. v.; unmindful, negligent, neglectful; heedless, careless, thoughtless; perfunctory, remiss; feebleness etc. 575.    inconsiderate; uncircumspect^†, incircumspect^†; off one's guard; unwary, unwatchful^†, unguarded; offhand.    supine etc. (inactive) 683; inattentive etc. 458; insouciant etc. (indifferent) 823; imprudent, reckless etc. 863; slovenly etc. (disorderly) 59, (dirty) 653; inexact etc. (erroneous) 495; improvident etc. 674.    neglected etc. v.; unheeded, uncared-for, unperceived, unseen, unobserved, unnoticed, unnoted^†, unmarked, unattended to, unthought of, unregarded^†, unremarked, unmissed^†; shunted, shelved.    unexamined, unstudied, unsearched^†, unscanned^†, unweighed^†, unsifted, unexplored.    abandoned; buried in a napkin, hid under a bushel.    ... meer laten zien