Idiot plot is a term that means your conflicts depend on your characters neglecting to ask each other simple, obvious questions that would clear everything up immediately.
At the moment in your plot when it becomes statistically inevitable that someone would bring up this issue, make it work for you instead of sweeping it under the rug. Have someone deliberately lie, or be wrong about that thing.
If you just fly it that they don’t notice that thing or that it doesn’t occur to them, then they are idiots.