If you describe something as unseen, you mean that it has not been seen for a long time.
a spectacular ballroom, unseen by the public for over 30 years.
公众30多年来未曾见过的豪华舞厅。
You can use unseen to describe things which people cannot see.
For me, a performance is in front of a microphone, over the radio, to an unseen audience.
对我来说,表演就是在麦克风前,通过无线电,传播给看不见的听众。