The sereno (the word is not, historically, spelled with the letter ñ) was a traditional Spanish-Mexican watchman. They would patrol the streets of towns at night, calling from time to time the hour and the state of the weather.
Frederick A. Ober, who encountered serenos in the smaller towns of 19th century Mexico, describes them as 'policemen furnished with lanterns', and that their calls are 'always ending up with 'Tiempo seréno,' or, 'All serene.' From this the mischievous Mexican youth have nicknamed him the Sereno...'
(Travels in Mexico)