It means conducted with secrecy, like a love affair or a CIA operation
[DOC] [TLE] The CIA and Covert Operations: To Disclose or Not to Disclose ... [PAR] May 1, 2002 ... leaders like George Washington (and almost every president to follow) secretly ... 
1997) (The need for secrecy in conducting diplomatic affairs was ... paramilitary 
operations as a means to secure German defeat.