Sometimes, the symbol is prominent, in the fashion of the Boros Truefire Paladin.
Truefire Paladin | Art by Michael C. Hayes
Sometimes, they're being formed out of pure magic, like the Simic symbol on Gyre Sage.
Gyre Sage | Art by Tyler Jacobson
Sometimes they're hidden, like the Gruul graffiti on the wall next to Wasteland Viper.
Wasteland Viper | Art by Lucas Graciano
Sometimes it's chiseled on the wall of an Orzhov mausoleum, as on Immortal Servitude.
Immortal Servitude | Art by Seb McKinnon
Sometimes they're graffiti on a person! Wait, that's just a Dimir tattoo. Anyway, here's Coerced Confession.
Coerced Confession | Art by Mathias Kollros