Sirius was not unobservant in his way, and he was canny on a number of topics. Watching what he said enough that he didn't get slapped or get a drink in the face not one of those. So the splash of liquid across his face and stinging his eyes was not exactly novel.
It was, however, unexpected, and he was left blinking in surprise, scowling. "Oi - what the fuck was that for?" he asked, sliding to his feet but not reaching to grab her. If she wanted to flounce off, he'd let her, but he was left more confused than really angry.