This maybe isn't the place to suggest it, but what about including the figurative meaning of "calling card" as well?
calling card, n.
... (figurative) an action or the result of an action by which someone or something can be identified: a dog whose calling card is a savage nip at the nearest ankles. http://www.bartleby.com/61/98/C0039800.html
NOUN ... 2. A distinguishing characteristic or behavior.
Examples off the web:
"Zorro often left his calling card carved on walls, furniture and even the clothing or skin of those foolish enough to challenge his lightning swift sword." http://www.tvacres.com/greetings_zorro.htm
"The killer left his calling card!" http://www.mysteryorbit.com/spiritchapter1.html
"the Greeks, the Romans, the Christian Crusaders, the Ottoman Muslims and assorted pirates, all leave a calling card along the aptly named Turquoise Coast." http://www.bootsnall.com/travelstories/me/mar...
As ken ashin said above, this is a somewhat cliched phrase, but nonetheless a very popular one which probably warrants its own LEO entry.
I'll leave it to others to suggest the best German equivalent - perhaps something along the lines of "Markenzeichen"/"ein Markenzeichen hinterlassen"?