Calling Card/Playing With

Basic Trope: A criminal's signature.

Straight: Alex leaves a card saying "the victim is infected" for every victim infected by his virus creation.
Exaggerated: Alex leaves several cards saying "the victim in infected" in extremely large font.
Downplayed: Alex merely leaves a footprint behind.
Justified: Alex has OCD, which compels him to leave something behind.
Inverted: Bob the police officer leaves a card saying "good to go" every time he finds a clue.
Subverted: Alex leaves a card behind that turns out to be blank.
Double Subverted: ... which says "the victim is infected" on the other side.
Parodied: Alex drapes a giant carpet saying "the victim in infected" over the area.
Zig Zagged: Sometimes Alex leaves a card, sometimes he doesn't.
Averted: Alex doesn't leave anything at all behind.
Deconstructed: Alex is immediately arrested.
Enforced: "But if the villain doesn't leave something behind, how will the hero get his first clue?"
Lampshaded: "How convenient of Alex to just leave a clue here..."
Invoked: Alex's right-hand man Manny convinces him to leave a clue behind.
Exploited: ???
Defied: Alex knows that leaving something behind would just be aiding his capture, so he doesn't leave anything behind.
Discussed: "There's the body. I'm pretty sure there's a clue around here somewhere..."
Conversed: "You ever wonder why villains keep leaving some sort of item behind wherever they go?"