Tungsten is 1.0015 times denser than gold. A gold-coated tungsten ingot could be adjusted (tiny internal cavity) to have exactly the same mass as a solid gold ingot of the same volume.

I've no idea what checks are done to exclude such artifacts from gold storage. If somebody accepted them and stacked them in a reserve vault, they might go decades without discovery.

I guess it's not something to worry about. It's improbable, and even so, if everybody agrees on the value of undetected bogus W/Au ingots, makes no difference from the real thing.