Journals were not necessarily very secure. Frank rarely wrote of anything of importance in his, and he kept it hidden and protected by charms. Of course, there was no way that Sirius could know that, and even if he did, no reason not to suspect that someone had used another journal to impersonate him somehow, without the real Frank's privacy even being compromised. Frank could not know for certain that none of that had happened to Sirius, either, but they'd both passed the first test. So far, so good.
"Hope you don't mind getting them to go," he said, smiling. He shifted forward until he was next in line, and absentmindedly ruffled his hair. He didn't make a conscious effort to appear unassuming in public, it just happened. Years of practice built on something that was already rather true to his nature. "I like some fresh air with my food."
Then he stepped forward and ordered his meal, before stepping aside to wait for it and let Sirius order. "My treat, by the way," he added. He had no idea what the boy's financial situation was, but who didn't like free food?