Twin Dragon Coin
A signal coin bearing twin dragons coiled around each other. It is a very simple, yet effective magical item. If the dragon side of the coin is touched to someone's skin, even briefly, they are marked. When the owner holds the coin they will feel a subtle tug in the direction of the one they have marked.
It wears off over time (months) and only one mark can be placed at a time. A skilled wizard could potentially find and remove the mark.