Synonyms and related words :
back-door, backstairs, clandestine, covert, cowering, cringing, doggo, feline, furtive, hidden out, hidlings, hole-and-corner, hugger-mugger, in ambush, in hiding, in the wings, lurking, on tiptoe, privy, prowling, pussyfoot, pussyfooted, quailing, quiet, shifty, slinking, slinky, sly, sneaking, sneaky, stealing, stealthy, surreptitious, under cover, under-the-counter, under-the-table, undercover, underground, underhand, underhanded, unobtrusive, waiting concealed