calling card
1. A small card containing one's contact information; a business card that may also be used for social purposes. I'm old school, so I like to hand out a calling card when I'm saying goodbye to a new acquaintance.
2. An item left behind by someone to signify that he or she had been present in a particular location. It was evident that the string of murders were committed by the same man, as he left his calling card, a single red rose, at the scene of each crime.
3. A special skill or talent that sets an individual apart from others. Primarily heard in US. His ability to paint such lifelike portraits really is his calling card in the art world.