How disappointing that would be. You'd probably hurl the book across the room and vow to never, ever read another book by the same writer. Part of the deal when reading a mystery is that the clues are there so you can get to the answer before the detective, and that the answer (which you will probably have got wrong) will have been staring you in the face all along.
Even if you're not writing detective fiction it's important to keep all your main characters going. They won't be centre stage all the time, but they need to be present within the narrative - even if it's just someone looking at a photograph or letter and having a fleeting thought about them.
Sometimes, of course, you want characters off stage - within my genre, for example, it's not unusual for a main character to be transferring her affections from one person to another, so it would be natural for there to be fewer thoughts about the past love and more about the new one, and equally natural for the main character to have a 'goodness, I haven't thought about Jim for ages,' moment.
But if the character is a major player then they need to be around. In Adultery for Beginners, for example, Adam doesn't turn up as a major character until the last section, but I put him into several scenes beforehand, even though Isabel (and the reader) hardly notices him. In Nice Girls Do Will goes off-stage for ages as he's stuck at Templecombe while Anna is distracted by Oliver in London, so I had to devise ways to keep him present in the narrative - a phone call, a letter. So, don't cheat the reader. Keep all your major characters to the forefront of the stage.
(There's another, very practical, reason for not losing characters along the way, but as this is getting a bit long, I'll write about that tomorrow.)