The hacker entered the system without anyone noticing.
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From the Yuan play 《冤家债主》 by an anonymous author: 这烦恼神不知鬼不觉 - the worry is known to neither spirits nor ghosts. The image: if not even gods or ghosts notice, no living person stands a chance.
usage
Typically used adverbially with 地 before a verb describing a stealthy action (sneaking, swapping, stealing). Mostly negative or mischievous in tone.
