unannounced

un- +‎ announced

unannounced (not comparable)

1. Not announced beforehand.
   • 1948 May and June, O. S. Nock, “Scottish Night Mails of the L.M.S.R.—3”, in Railway Magazine, page 157:

     But soon one notices, first in ones and twos, and then in larger groups, men who are very much alive at this hour in the morning, the sorters for the Caledonian, and Edinburgh sections of the "Down Special"; mail is piled high on the platforms, and the postal men glance occasionally towards the platform intermediate signals. By this time the calling-on arm is pulled off, and a few minutes later in comes the train, unannounced by the loud-speakers, to the mystification of many travellers.

   • 2015 April 15, Jonathan Martin, “For a Clinton, It’s Not Hard to Be Humble in an Effort to Regain Power”, in The New York Times‎:

     In 2008, she was at times criticized for being detached from voters, insulated in a bubble of staff and security and avoiding spontaneity. She began her 2016 campaign by riding 16 hours in a van from New York to Iowa, making unannounced stops at gas stations and fast-food restaurants, before arriving for what was billed as a series of low-key conversations with a handful of voters.

   • 2023 October 18, Nick Brodrick, “The grand gateway to Glasgow”, in RAIL, number 994, page 33:

     The Major Stations category is keenly contested, with 24 other large city centre termini and major rail hub stations all vying for recognition. As ever, the NRA's expert panel of judges visited each as unannounced mystery shoppers.