While Mr. Utterson and Mr. Enfield were walking, what did they see that reminded Mr. Enfield of an odd story?

The answer to this question about the door in The Strange Case of Dr. Jekyll and Mr. Hyde is a very simple one: While Utterson and Einfield were on their usual Sunday walk in a neighborhood that had a prosperous business street that looked welcoming and cheerful, they passed a door on a building that had no window and was in need of repair of long neglect. The door itself was unwelcoming, in a welcoming neighborhood, with no bell or knocker on it:

The door, which was equipped with neither bell nor knocker, was blistered and distained. Tramps slouched into the recess and struck matches on the panels; ... the schoolboy had tried his knife on the mouldings;

The door, and the building wall into which it was set under a greatly sinister thrusting gable, was a great point of contrast to the rest of the neighborhood. The neighbors all took pride in and care of their buildings and shop fronts so that they looked inviting to shoppers. Yet in their midst stood a long unheeded door that fairly shouted of the neglect and disuse of a full generation.