It's not easy for somebody to get into the building unobserved.

进入这栋大楼而不被发现很不容易。

Looking around to make sure he was unobserved, he slipped through the door.

他四下看看，确定没人看见他，就从门里溜了进去。

He took his seat in an unobserved corner of the assembly room.

他在会议室一个不为人注意的犄角坐下来。

With only one observation per country, it is impossible to control for unobserved heterogeneity across countries.

由于每个国家只有一个观测结果，因此不可能控制各国间未观察到的异质性。

If it's unobserved, it will.

如果观测不到，那它会通过。