That's according to a report from the Wall Street Journal, citing people familiar with the probe, which has been underway since at least last fall.
The news comes just over a week after Sandberg announced her departure from the company after 14 years.
Meta and Sandberg did not immediately respond to Reuters' requests for comment.
Among the activities being investigated is the work of Meta employees to support Sandberg's foundation, Lean In, according to the report, and the writing and promotion of her second book.
Meta's chief growth officer is set to take over as chief operating officer although CEO Mark Zuckerberg said he did not plan to replace Sandberg's role directly within the company's existing structure.
Sandberg, whose close partnership with Zuckerberg powered the growth of the world's biggest social network, said she will continue to serve on Meta's board after leaving the company in the fall.
