I once heard that a building either at or near Croydon Airport has the
word NO written on so as to advise pilots that they are not
approaching Gatwick.

There used to be a gasholder at South Harrow with NO painted on the
side to indicate that it was near Northolt airfield and not on the
approach to Heathrow.


That was done after someone tried to land a 747 at Northolt.