evidense
Glaringly-obvious proof that is so easy to observe that even a clueless "thickhead" should be able to notice it.
In Edgar Allen Poe's "The Purloined Letter", Monsieur Dupin is able to locate the stolen missive quite easily in the Minister's study, even though the Prefect and his men had twice minutely searched said room, yet never noticed said evidense that was right out in plain view.