He'd had quite the dream the previous night. So much tension and duality. Percy wasn't exactly trying to be like he was in the dreams, but it had been on his mind, to say the least. He'd found himself staring out the window or staring at his computer as the day had gone on. Everything was roiling through his mind, how his dream self was becoming more enveloped in the dual life he led. Or, well, triple life he led if one counted the persona he used to infiltrate Chauvelin and Robespierre's ranks.
"Uh...hi. Sorry, totally didn't mean to sing, it just happened." Though he had to say it was a catchy tune.
It would seem he had caught the singing bug because he certainly hadn't known this song before.