http://www.businessinsider.com/mystery-why-did-glds-published-list-of-gold-bars-shrink-2009-10

Nothing makes for a good market mystery like gold, especially gold socked away in the vaults.

So here's a good one, via FT Alphaville, from Professor Antal E. Fekete. Actually, it's a double dose.

We must guard ourselves against falling victim to the rumor-mills, while keeping our eyes
peeled for the very real possibility that the growing shortage of physical gold can no longer be papered over with paper gold (pun intended). Another story is about GLD, a leading gold
ETF, which publishes its bar-list every Friday at the close of business, reporting the serial
number of every bar in inventory. The list is customarily well over a thousand pages long.
But, lo and behold, on Friday, October 2, and on Friday, October 9, the bar-list shrank to a
mere couple hundred pages, with no explanation offered. Could it be that the management of GLD has taken a bribe, and replaced physical gold in inventory by paper gold, in order to save the face and skin of the bullion banks that have gone naked short and subsequently got cornered?
If so, it won’t get very far. The leadership of the US House of Representatives may
well be able to put in deep freeze the motion of Dr. Ron Paul, seconded by over 250 other
congressmen on both sides of the aisle, to audit the Federal Reserve, but it has no power to stop the auditing of the ETF’s or bullion banks as required by contract law. According to
some reports independent auditors, at the insistence of parties holding expired forward
purchase contracts to deliver gold, are descending on ETF’s and check their vault’s contents against their books. The noose is tightening around the neck of fraudulent banksters caught in the short squeeze.

And then here he talks about the so-called "Archimedian Test"

Reports are circulating that similar audits of certain Asian depositories have already produced “good” delivery bars (400 oz or 12.5 kg gold bricks) that have been gutted and stuffed with tungsten — a metal whose specific weight approximates that of gold, so that the famous test of Archimedes (fl. 287-212 B.C.) based on the Law of Buoyancy, designed to expose fraudulent goldsmiths, would be inapplicable. Isn’t it strange that criminal law punishes the fraudulent stuffing of gold bars, but allows the stuffing of gold assets in the balance sheet with paper gold? After all, the specific value of tungsten is much higher than that of paper!