お先に失礼します osaki ni shitsurei shimasu

Shitsurei means a lapse of manners, so the phrase you say on your way out of the office is, word for word, an apology for the rudeness of leaving before other people. Somewhere in there is the whole history of Japanese working hours.

In practice it is a courtesy, not a confession. You say it, the room answers otsukaresama, and the ritual releases you cleanly instead of letting you slink off. Younger workers, remote schedules and a decade of pressure on overtime have loosened the guilt considerably, though the words held on, now typed into a chat window at six with a small wave. Leaving on time is normal. Announcing it is still polite.