The pillow smudge went by unnoticed, for now, anyway. Undoubtedly James would be painfully aware of his dirtied pillow when he goes to lie down. Hopefully, however, James will notice the mud stains before he makes to lie his head down on it rather than after.
"Exactly," James said once the metaphorical light in Sirius' mind went off. "We could use it to tell each other things when we're not right by one another, things that we don't want everyone else to see," James said. "Things that we don't want McGonagall to see," James added. "Because it would work against professors too, right? That's at the very least a possibility, yeah?" James asked Remus. Oh, think of all the things he could write and do without knowing McGonagall was watching him! The freedom! These journals suddenly didn't seem so oppressive anymore. They now seemed like a means for instigating mischief and chaos, a device they could use to plan things.
"No she told me I had to leave," James said. "But that doesn't mean she's out there now, we could sneak by later and do something," he pointed out and then after a moment's pause to think he broke into a mischievous smile. "I think I have an idea," he told them both.