WordNet sense 1 (relating to the peripheral and unimportant aspects of life):

hole-in-corner

WordNet sense 2 (not important or noteworthy):

insignificant, unimportant

WordNet sense 3 (conducted with or marked by hidden aims or methods):

clandestine, cloak-and-dagger, hush-hush, secret, surreptitious, undercover, underground, hugger-mugger

WordNet sense 4 (secret or hidden; not openly practiced or engaged in or shown or avowed):

covert