Historical clue formulations

• Surreptitiously — 2018-06-13
• Sub rosa — 2017-03-17
• Surreptitiously — 2009-07-15
• Furtively — 2009-06-29
• Sneakily — 2005-11-16
• Behind one's back — 2004-06-04
• Furtively — 2004-04-12
• Furtively — 2000-04-18
• In secret — 1998-05-23