Furtiveness is the noun: the quality, or the air, of being furtive. You might speak of the furtiveness of his glance, or say there was a furtiveness about her as she left. It names that secret, slightly guilty manner as a thing in itself. It is much rarer than furtive or furtively, and it sounds a little formal — the sort of word you meet in a novel or a careful report, not in the playground. Most of the time, the adjective is the word you actually need. Though actually, when a writer does choose the noun, they are often pointing at a pattern, not one look: a habit of hiding, a lasting air of not wanting to be seen.

鬼鬼祟祟、偷偷摸摸的樣子