Read MoreSEC investigating whether activist funds colluded: Report
Which firms the SEC is investigating was unclear.
Hedge fund managers likely don't have to tell their clients of an initial government letter requesting information—which some got, the Journal reported—according to hedge fund investors and industry lawyers. Firms are only likely to tell their clients of an inquiry if it's material to their business. An SEC investigation is a civil matter, not a more serious Department of Justice criminal probe.
Money managers are allowed to discuss trade ideas before investing, but they are required to disclose that they are acting in concert if their combined stake in a stock totals 5 percent or more. Another common trigger for a group disclosure is when an investor files a proxy statement in an attempt to change something at a company through a shareholder vote.
Recent hedge funds to team up publicly include Pershing Square Capital Management and Sachem Head Capital Management on Zoetis; Barington Capital Group and Macellum Advisors on Children's Place; and Corvex Management and Related Funds Management on Commonwealth REIT.
Having multiple hedge funds in the same stock doesn't always mean coordination.
Trian Fund Management and Marcato Capital Management both invested in Bank of New York Mellon with different views on the future of the company, for example, and Barington and Starboard Value appear to have unintentionally bumped into each other on Darden Restaurants.
"When a company is underperforming, it makes sense multiple activists will be involved," a manager told CNBC.com.
The person added that companies were often sensitive to being targeting by groups, making activists extra careful legally: "We make sure we cross every 't' and dot every 'i.' "
Major activist firms either declined to comment or did not respond to requests for this story, including Pershing Square, Third Point, Elliott Management, Sachem Head, Starboard Value, Trian, Jana Partners and Marcato.