Unobserved means nobody was watching — no eyes on you, so nothing you did was seen. It is close to unnoticed, but there is a real difference worth learning. Unnoticed is about not registering: someone might have looked straight at the thing and their mind did not catch it. Unobserved is about no watcher being there at all, or about escaping the watcher who was. That is why unobserved so often goes with movement and with doing something quietly: he entered unobserved, she left unobserved, the letter was taken unobserved. It has a slightly formal, written feel — you meet it in reports, in crime stories, in science ('the animals behave differently when they are unobserved'). In everyday speech people say 'without anyone seeing' or 'when nobody was looking'. It sits after the verb far more often than in front of a noun.

沒被看到的、無人在旁看著的。日常中文就是「趁沒人看到」「沒人在旁邊盯著的時候」。和 unnoticed 的差別：unnoticed 是「有看到但沒發現」，unobserved 是「根本沒人在看」。