Shadowmarks are a tool for the Thieves Guild to identify the worth of a target home. Some are protected, some are worthless, others have loot, and still others might even be dangerous. If you've ever wondered why the five potential player homes in the major cities didn't have these markings, wonder no more. Now, instead of being the only homes in a city missing them, they will be marked according to their worth.

If the player does not yet own the property, it will be marked empty.
If the player owns the property, it will be marked as containing valuable loot.
If the player is a member of the Thieves Guild AND the city has been brought under the influence of the Guild, then it will be marked accordingly as being owned by a guild member.

The marks should properly update themselves throughout the game as the player acquires the houses and joins the guild.