Warren Buffett’s Berkshire Hathaway is buying another mystery stock under confidential treatment, allowing the conglomerate to quietly build up a position and limit price movement without stoking volatility. A new regulatory filing disclosing its investment portfolio Thursday evening revealed that Berkshire has omitted one or more holdings “for which it is requesting confidential treatment from the U.S. Securities and Exchange Commission.” The SEC typically grants such confidential treatment when the disclosure of an ongoing investment strategy by any asset manager with more than $100 million, “would impede competition and could cause increased volatility in the market place.” Due to the sheer size of Berkshire’s equity portfolio, recently standing at some $275 billion worth of stock, any bet would usually have to amount to billions of dollars in order to have an effect on returns, and that could easily take months to build. Keeping a new stake confidential for a time is beneficial for Berkshire as it limits volatility until it’s done buying. News of Berkshire’s new stock picks typically leads other investors to follow suit, quickly moving the share price. The latest request for confidential treatment could have been the work of one of Buffett’s investing lieutenants, Todd Combs or Ted Weschler, especially after Buffett’s decision to step down as CEO at the end of this year. The 94-year-old legend recently revealed he has finally started to feel the effects of aging, such as occasionally losing his balance. Past secret stocks In late 2023 and early 2024, Berkshire received confidential treatment for a few quarters to mask its purchase of Swiss insurer Chubb. Berkshire now owns about $8 billion of the stock. Similarly, when Berkshire bought stakes in Chevron and Verizon in 2020, it also kept those stakes under wraps until it finished building the positions. Berkshire’s first-quarter earnings report offered little clue as to what the new purchase could be. The 10Q filing said that the cost basis for equity securities in “commercial, industrial and other” increased by about $2 billion last quarter, while investments in “consumer products” rose by $1.1 billion. Here’s what we already know about the first quarter: Berkshire doubled its stake in booze stock Constellation Brands , while hiking its bets on Domino’s Pizza , swimming pool supply distributor Pool Corp. and aerospace and electronics contractor Heico .