The Watchman was a random event that appeared during Thieving, similar to other guardian events such as the Rock Golem. His level differed depending on the player's combat level. He was removed due to players' complaints about being killed too quickly when pickpocketing at a low life point level and wearing no armour. The Watchman would say "(Player's Name), you are under arrest!" before attacking.
There was a bug in free worlds where if the player tried to steal from a stall (such as the tea stall) in south-east Varrock, a watchman would appear. He would attack and the player could fight him. He would not drop any member items on free to play worlds.
In the Draynor Bank's security recording of the Draynor Bank Robbery, a Watchman appeared but was killed quickly by the Wise Old Man. When the watchman was discontinued, the watchman in the video footage was replaced by the Pillory Guard, even though the bank robbery was an event that had supposedly happened before the Pillory Guard existed.