A "surprised expression" refers to the way someone's face looks when they are suddenly taken aback or startled by something unexpected. Full definition
(Yet when one ventures to make this claim in the presence of theologians, one is invariably met with a slightly surprised expression, followed by an assenting, «Of course.») (religion-online.org)
Digital images of hyperreal female body parts stick out of white foam, shiny female heads with surprised expressions fall through white cushions. (aqnb.com)
His heavy eyebrows seem to rest in a permanent arc, which along with hair that pushes up in front gives him the surprised expression of someone forever asking, Could you repeat the question? (si.com)