Sirius was at work at the Broomsticks when the cat appeared to slink around his ankles. There were always cats around Hogsmeade, but he didn't typically allow them behind his bar- or anywhere near him if he could help it- and so he bent to pick the thing up and toss it out, only registering the fact that it was more silver wisp than real cat once he was crouched low. Which fortunately meant that it could whisper its message to him without being heard, or even noticed, by the patrons.
Straightening as the patronus disappeared again, he rubbed a hand through his hair and yelled to Rosmerta that he was going to need to switch shifts on Thursday.