stealthy
STEL-theeadjMeaning moving quietly and carefully so as not to be noticed
Memory hook STEAL-THY: steal past without a sound. Jonah tiptoes past the sleeping dog in his socks, carrying his shoes; he steals by so quietly that he is stealthy.
The stealthy cat crept up on the butterfly without rustling a single leaf.
In context Unlike his noisy brother, who stomped up the stairs, Jonah was stealthy, slipping out the back door so quietly that even the dog did not wake.
Similar sneaky, quiet, secretive, sly
Opposite noisy, clumsy, conspicuous
From stealth, which comes from the same old English root as steal.