What is the word for the look on someone's face when not being observed?

There is no specific Word for this, however, there are several terms that can be used.

In Photography, the term is 'candid'--when a person does not know that the picture is being taken. This term refers to photos taken without staging or posing.

When a person is not being observed, they tend to be more relaxed. They may not think about their expressions and actions as much, leading to a more random type of behavior. You could call them 'unaware'.