Is There a Resolution at the End of a Crime Series?

I think there has to be some sort of resolution, otherwise the reader is profoundly disappointed at the end. Often that's with the detective, isn't it? Theyre a sort of un unresolved relationships between the detective and colleagues. And part of the reason i got tangled up in the third in the series is i was writing about people trafficking - which is a theme that goes acrosste the previous two books. The first one's under a silent moon, and the second one is behind closed doors. It wasn't that it was an end to the louisa smith series, because i'm sure that let me nor so many more things i could write about with her and her team.