Okay. So I am writing the third novel in a series and in this novel the main story arc is the apparent death of one of the MCs (in reality he has been kidnapped, but only one person believes this.) So the main arc is my other MC trying to find him. Do I have to resolve this in this book? I wasn't planning on "finding" him until the next book. There is plenty of other stuff going on. I know the book has to have an ending that leaves the reader satisifed, but do I have to resolve this issue thus completing the arc? Help.
It's hard to say without reading the novel. It depends on the "plenty of other stuff going on," and how much the kidnapping plot takes a backseat to these issues. If the kidnapping plot is the main plotline rather than just a subplot, I can sense a problem.
For example, I'd be much more satisfied with an ending where the characters, who have believed all along so-and-so is dead suddenly proclaim, "He's alive!" I have a clear idea where the next book is going.
If, however, the MC has been searching the entire novel for another missing character and they never meet up, I might be upset. It's all dependent, though, on how many clues are laid out, how it affects the other plots, and what it means to the story as a whole if/when the character is found, and how far into the search the characters are.