C leaves calling cards at the scene of his crimes(literal cards, not a reoccurring sign that is special to the culprit, as "calling card" is usually used as), and disappears without a trace.
Now, do we know of anyone in league lore that uses cards, robs people, and has a tendency to teleport away if given 2 uninterrupted seconds? Who had planned to do jobs in Piltover? Who also likes robbing rich people by sneaking into their places and escaping without any obvious entrances or exits?
Her services were soon requested by Demacia to help track down a mysterious outlaw who had begun committing high-profile heists.
The bandit, who always left a card with an ornate 'C' at the scene of the crime, became Caitlyn's arch-nemesis. To this day, Caitlyn still searches for this cat burglar, and the chase has led her all across Valoran.
and from burning tides...
I duck into a blind alley opposite the warehouse. It’s a dead end, and it’s not as dark as I’d have liked. If I’m still here when the patrol comes back, they will see me. And if they get ahold of me, the best I can hope for is a quick death. More likely, I’ll be taken to him... and that would be a far more painful, drawn out way to go.
The trick, as always, is not to get caught.
Then I hear them. The bruisers are returning early. I have seconds, at best. I snap a card from my sleeve and weave it through my fingers; it’s as natural as breathing. This is the easy part, the rest can’t be rushed.
I let my mind drift as the card starts to glow. Pressure builds around me, and I’m nearly overcome with the promise of everywhere. Half-closing my eyes, I focus, and picture where I need to be.
Then, there’s the familiar lurch in the guts as I shift. A displacement of air, and I’m inside the warehouse. Gone with barely a trace.
Damn, I’m good.
One of the Jagged Hooks outside might glance up the alley and notice a single playing card falling to the ground, but probably not.
I mean, it sounds similar. And honestly, what kind of robber purposely leaves a trace of himself behind? Unless, of course, that is the only way he could get out...
Based on evidence, the "C" looks like it would be Twisted Fate, the Card master.