His Honor the Lord Mayor was naturally quite interested in the adventurers who took temporary residence in one of the finer inns of the town — especially when he learned that one of their aims was to find the local assassins’ guild and settle an old score with it. The guild had caused the Lord Mayor many problems in the past, and he welcomed the possibility that the guild might soon suffer problems of its own. The adventurers, however, were close-mouthed and were cool toward his offers of assistance. His course of action was to rely on the capable services of Ferd, his halfling informant. Ferd began to tail the adventurers whenever they ventured into the city reporting back to the Mayor on their activities.
At this point, the DM creating this scenario must pause. Obviously, the game is going to develop in different ways depending on whether or not the adventurers notice that a nondescript halfling is following them around. How should he determine if the characters notice or not?
So, using the B/X rules how would you handle the likelihood of the party spotting the halfling?