When a person casts a corporeal Patronus for the first time, a mark in the shape of their Patronus appears on their soulmate's body. Playboy Sirius Black never expected to get one.

"I've got you under my skin
I tried so not to give in
I said to myself, 'this affair never will go so well'
But why should I try to resist when, baby, I know so well
I've got you under my skin"

Sirius Black never expected to get a soulmate mark. He wasn't sure he was cut out for monogamy, for starters. And the kind of birds he preferred to spend time with weren't the kind who spent time mastering complex magic like the Patronus Charm. In fact, he was pretty sure none of his past paramours had earned a Charms O.W.L., let alone studied it at the N.E.W.T. level, which was when the charm was taught at Hogwarts. He, of course, had mastered it on his own years ago, and he'd never seen a sign of a shaggy black dog on anyone he'd been with, nor heard of anyone with such a mark.