“13” or “Thirteen” (the boss is around—watch out)

 
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23 May 1933, Courier-Post (Camden, NJ), “On Broadway” by Walter Winchell, pg. 10, col. 4:
Code.
A Hollywood soda-jerker forwards this glossary of soda-fountain lingo out there…“Shoot one” and “Draw one” is one coke and one coffee…“Shoot one in the red!” means a cherry coke…An “echo” is a repeat order…“Eighty-six” means all out of it…“Eighty-one” is a glass of water…“Thirteen” means one of the big bosses is drifting around…A “red ball” is an orangeade…“Squeeze one” is a limeade…“Eighty-nine” means that a movie player of importance is in the store, and “Twisted, choke and make it cackle!” means a chocolate malted milk - with an egg in it.
 
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29 March 1936, Helena (MT) Daily Independent, “National Youth Administration Compiles Lexicon of the Soda Jerker After Five Months Work” by Helen Dallas, pg. 2, col. 7:
“Eighty-one” for water, “forty-one” for orangeade and “thirteen” to indicate that one of the “big bosses” was drifting around were taken as examples.
 
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17 September 1971, The Messenger (Madisonville, KY), “‘Hash-House Greek’ Colorful U.S. Slang” by Dan Carlinsky (New York Times), pg. 6, col. 3:
thirteen. The boss is around—watch out.
 
Newspapers.com
27 September 1981, The Herald Statesman (Yonkers, NY), “Counterwailing” by Dan Carlinsky, Suburdia Today, pg. 18, col. 4:
13. Watch it, the boss is around.
 
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10 July 1985, The Times-Mail (Bedford, IN), “Soda jerk unique on American scene” by Ink Mendelsohn, pg. 5, col. 5:
THIRTEEN—One of the big bosses is around