My grandparents, Anne Bauer and Norman McLeod Dingman, married secretly in 1916.
Anna Fredericka Bauer and Norman McLeod Dingman
Anne, born Anna Fredericka Bauer probably on 29 April 1891 in Keyport, New Jersey,  graduated from Presbyterian Hospital Nursing School in New York in 1914.  After graduation, Anne was a head nurse at Presbyterian Hospital until 1915, when she went to Germany to serve with the Red Cross during World War One. 
Norman McLeod Dingman, Anne's husband-to-be, received his Doctor of Medicine degree from Columbia University on June 2, 1915.  The two apparently met during Norman's medical internship.
My mother gave me a little book that had belonged to Anne which reprints a commencement address.  In it, Mom inscribed information about her parents.
Inside front cover, Commencement address for Presbyterian Hospital Training School for Nurses, 1907, containing Anne Bauer's signature and an inscription by Nancy Dingman Watson. 
You can see from her signature and date that Anne apparently acquired the book in 1914. However, this address was published in 1907. Mom reported that it was the address at Anne's graduation; but in 1907 Anne was only about 16, and didn't graduate from nursing school until 1914.
According to my mother, Anne and Norman's daughter, Norman interned at Columbia-Presbyterian,  but on his World War I draft card he reported that he was an intern at St. Luke's Hospital.  In any case, the two hospitals were closely associated.
Anne and Norman were married on 3 October, 1916, in Culpeper, Virginia at St. Stephens Episcopal Church.  Why a secret marriage? The story goes that Columbia University medical interns were not allowed to marry. I was not able to find any information about this rule. After they married, they kept the fact hidden, at least from Anne's parents. In the evenings at her home in Keyport, Anne's sisters apparently spirited Norman upstairs to her bedroom after the parents had gone to bed. How long this subterfuge could have lasted is anyone's guess!
Wedding announcment for Anna Bauer and Norman Dingman 
A year later the marriage was officially announced, but no attempt was made to hide the date of the marriage. 
1. No birth record has been located for Anna Bauer. The State Archives of New Jersey could not locate a record between 1890 and 1900. Anna is listed as age 9 on 1 June in the 1900 Federal census. 1900 U.S. census, Monmouth County, New Jersey, population schedule, Keyport, enumeration district (ED) 123, p. 43B (stamped), p. 13B (penned), dwelling 307, family 333 [dwelling and family changed from original writing: several crossings out and changes], Herman O. Bauer; digital image, Ancestry.com (http://www.ancestry.com : accessed 14 May 2017); citing NARA microfilm publication T623, roll 986. Her death certificate, for which the informant was her husband, gave her birth date as 29 April 1891. Ho-Ho-Kus, Bergen County, New Jersey, death certificate, state file no. A0009314443, Anna Fredericka Dingman, 1955; Borough of Ho-Ho-Kus, New Jersey, Office of Vital Statistics. On the marriage license for Norman Dingman and Anna Bauer her age was reported as 24. If born in 1891 she would have been 25 in September 1916. See Culpeper County, Virginia, certified copy of a marriage certificate (1916), Dingman–Bauer; original is at Culpeper County Circuit Court Clerk's Office, Culpeper.
2. Presbyterian Hospital (New York, N.Y.), The Presbyterian Hospital in the City of New York: Forty-Sixth Annual Report for the Year Ending Sept. 30 1914 (New York: Trow & Smith Manufacturing Co., n.d. , 76.
3. "The Red Cross," The American Journal of Nursing XV , (June 1915): 746; image copy, Google Books (https://books.google.com : accessed 21 November 2017). Also, Nancy Dingman Watson, inscription on the inside cover of Henry Fairfield Osborn, Sc.D., LL.D., D.Sc., Science and Sentiment: An Address Before the Graduating Class of the Presbyterian Hospital Training School For Nurses (New York: [n.p.], 1907. Ms. Watson was the daughter of Anne Bauer. Watson stated that her mother was "head surgical nurse." The Red Cross article says she was head nurse. Further research is necessary.
4. Henry Fairfield Osborn, Sc.D., LL.D., D.Sc., Science and Sentiment: An Address Before the Graduating Class of the Presbyterian Hospital Training School For Nurses (New York: [n.p.], 1907.
5. Watson, inscription on the inside cover of Osborn, Science and Sentiment.
6. Columbia University, New York, Annual Commencement program for 1915, p. 9; Google Books (https://books.google.com : accessed 17 November 2017).
7. Watson, inscription on the inside cover of Osborn, Science and Sentiment.
8. "U.S., World War I Draft Registration Cards, 1917–1918," images, Ancestry.com (https://www.ancestry.com : accessed 21 November 2017), Norman McLeod Dingman, Registration Card No. 25, Draft Board 2, Rockland County, New York; citing World War I Selective Service System Draft Registration Cards, 1917–1918, NARA microfilm publication M1509, [no specific roll cited].
9. Culpeper Co., Va, certified copy of a marriage certificate (1916), Dingman–Bauer.
10. "Kept Wedding Secret More Than Year," Asbury Park (New Jersey) Press, 1 October 1917, p. 2, col. 4; digital image, Newspapers.com (https://www.newspapers.com : accessed 29 November 2017).