A character is depicted without a navel, even though it should be visible because the midriff is exposed.
In case of humans and crossovers with mammals (cat girls, vampires) this is usually a mistake by the artist. However, sometimes it is used on purpose to highlight the non-human nature of the character (aliens, androids) or the fact that they have been produced by some kind of unnatural means.
A character not having a navel is not an error if it is a crossover with a non-mammal (mermaid, harpy) or some kind of other fantasy creature (dragon, fairy) that possibly hatched from some kind of egg.
