Lost in his own thoughts, as he was, Severus didn't notice the woman's approach. He wasn't normally so unaware of his surroundings -- except when he was drugged, it seemed -- because that could be a fatal error for a spy. Something about that lovely little blast from the past, however, overrode his normal wary instincts. At least temporarily. Being bumped into quite effectively jarred him out of his thoughts, and he wheeled about with a sharp intake of breath.
He'd started to go for his wand, as well, on instinct, but at the 'excuse me', he managed to stand down from the mental red alert. He simply wasn't used to being touched at all, even accidentally. Especially not by strangers. And most especially not by pretty ones. "No, it's quite all right," he lied with a shake of his head and a semblance of a smile. She could no doubt tell from his reaction that it hadn't been quite all right, but that was hardly her fault. No harm done anyway, apart from getting his heart rate up a bit.