surreptitious

adj

1. Stealthy, furtive, well hidden, covert (especially movements).

   “He read the letter aloud. Sophia listened with the studied air of one for whom, even in these days, a title possessed some surreptitious allurement.”

   “It is also worth noting the case law on prisoners' correspondence which establishes that interception of a person's communications need not be surreptitious in order to amount to an interference with respect to Art 8 (1) [ECHR].”