He'd tried not to startle the man, but really he knew what it was to see someone transform into an animal and he was a wizard, how much more strange would it be as someone who didn't come from his world? And Sirius was fairly certain this man was not from his world. So when he caught sight of the knife in his hand he tried to keep his own hands in sight without grabbing for his wand just to make certain he didn't startle him any more than he already had.
"Yeah," He agreed sheepishly, "Yeah sorry, I -" He started and stopped shrugging. "It's easier sometimes." He admitted, especially after everything. He was trying of course, he spent time with Jamie, he and Regulus were well not getting along exactly, but he was attempting not to be openly antagonistic with his younger brother. But in a hotel filled with strangers, it could be overwhelming at times. So sure there were times he preferred wandering around as Padfoot.
"It's magic," He added, "I - where I'm from, I'm a wizard." He went on, explaining a little more, knowing he was breaking some of the basic laws of his world, but all things considered he didn't think he could be a bigger criminal in his world than he already was. To say nothing of the fact that no one there was going to have any clue he was talking to muggles about what he was and what he could do so it didn't really matter. "Sorry."