stealthily

salakähmäisesti, varkain In a stealthy manner, furtively.
1912: (w), (w), Chapter 5

So intent was he upon this personal appraisement of his features that he did not hear the parting of the tall grass behind him as a great body pushed itself stealthily through the jungle; nor did his companion, the ape, hear either, for he was drinking and the noise of his sucking lips and gurgles of satisfaction drowned the quiet approach of the intruder.