Argument from Ignorance
The argument from ignorance treats a claim as true because it has not been disproven, or false because it has not been proven. It exploits a gap in evidence as if the gap itself were evidence. Elizabeth Cox frames the mistake well through cases where accusation, suspicion, and burden-shifting do the work that evidence never did.
The fallacy usually works by shifting the burden of proof. Instead of the claimant having to support the accusation, everyone else is forced to refute it. That is why it becomes especially corrosive in cases involving secrecy, conspiracy, disloyalty, or hidden motives. The less falsifiable the accusation is, the more tempting this move becomes.
The cure is procedural as much as logical: ask who is making the claim, what evidence they have supplied, and whether the absence of refutation is being smuggled in as proof. A possibility is not a conclusion, and suspicion is not an argument. This is one of the core failure modes in critical-thinking-and-logical-fallacies.