Something about this notice clearly amused Jack — and it is not difficult to see why. Someone had gone to the trouble of making a proper sign to warn passers-by not to mistake what they were seeing for a fire.
Presumably the warning existed for a reason. Perhaps enough concerned members of the public had reported smoke or steam from the site that the owners eventually decided a permanent notice was the simplest solution.
Whatever the story behind it, Jack evidently thought the sign itself was worth preserving.