Warren Buffett just revealed a new stake in steel manufacturer, Nucor.Every quarter, institutional investment funds managing more than $100 million are required to file a form 13F with the Securities and Exchange Commission (SEC). These disclosures itemize which stocks the biggest names on Wall Street bought and sold during the most recent quarter.There is, however, a catch. From time to time, firms can request that certain positions remain temporarily confidential -- usually if the stake has not yet crosse ...