They did a study where they sometimes tricked dog owners into thinking the dog had done something they shouldn't do when the dog had in fact done nothing wrong. Basically the dog's expressions had nothing to do with anything, the owners behavior and conclusions of guilt/innocence are really the only factors.

Can dogs really look 'guilty'?

Researchers concluded that any such "guilty look" is a response to human behaviour and has no relation with the dog's actions or sense of having broken any rules.