Like underground gas chambers at Sobibor, covered up after the rebellion there.
A sunken German sub off Norway (plus other fighting equipment).
An RAF fighter, thought shot down in an airfight. (It was actually lost in the desert. More interesting items on this link, as well.)
Even a Nebraska soldier's medal, hidden inside a tree in Vermont.
Now a gold ring and necklace have been found, hidden in the false bottom of an enamel cup -- from Auschwitz. Staff at the Auschwitz museum in Poland noticed after the canvas bottom eroded, and exposed the jewelry.
The cup was one of the artifacts from the Nazi prison camp. Someone obviously planned to retrieve it...and never did.