The night goes late very late in Madrid, and there's not a day of the week that the streets aren't flooded by hordes of after-hours diners and bar-hoppers.
The diners are never seen at night in Madrid. 
mnli hypothesis: The diners are never seen at night in Madrid.  premise: The night goes late very late in Madrid, and there's not a day of the week that the streets aren't flooded by hordes of after-hours diners and bar-hoppers.