But the matter remained a tightly guarded secret from the public until the learned of it through a source in January.

Source: https://arktimes.com/arkansas-blog/2023/04/24/ua-system-board-says-no-to-resolution-supporting-university-of-phoenix-purchase

However, the identity of the person who gave her the perfume has remained a closely guarded secret.

Source: https://www.dailymail.co.uk/femail/article-12583507/Royal-woman-SIX-TIMES-rivals-expensive-scent.html?ns_mchannel=rss&ns_campaign=1490&ito=1490

It will remain a closely guarded secret, despite the clamour of requests from guests every year.

Source: https://www.dailymail.co.uk/femail/article-12844055/Jeffrey-Archer-Christmas-parties-Margaret-Thatcher-Donald-Sinden-Cilla-Black-Betty-Boothroyd.html?ns_mchannel=rss&ns_campaign=1490&ito=1490

Spruce Pine’s quartz is so valuable that, as the Vancouver-based journalist Vince Beiser observes in his book “The World in a Grain,” almost everything about it, outside of its purity, is a closely guarded secret.

Source: https://www.newyorker.com/magazine/2023/10/30/the-real-cost-of-plundering-the-planets-resources

Stafford noted: "It was very much a private visit a last minute closely guarded secret.

Source: https://www.irishcentral.com/culture/entertainment/bruce-springsteen-kildare-pictures