Coins are large gold medallions with the profile visage of a monkey stamped into them. Coins appear in Thief matches and are available as a collectible set within the Mapmaker of TimeSplitters Future Perfect. In a Thief match, a coin falls from killed players and killed bots, the object of the match being to collect the most coins. In Mapmaker Story or Assault modes, collecting the coin (as part of the collectible set) serves as a trigger for an action (e.g., display a message, complete a mission objective, unlock a door, activate an autogun, toggle a switch, etc.).
There is a mini-glitch with the coin placements in the mapmaker. After adding the coin as a trigger "new collectible set" (which is required if you want to place coins), simply delete the trigger and the coins cannot be collected. However, the coin placements will remain to decorate your map.