We have gotten slight glimpses of Phil being Phil when he thinks he's alone, during The Incident, and that time fairly recently where he left the camera on and had his arm hanging over the top of his head like a chimp, but there was no audio with the former and he said nothing during the latter.
I don't think he actually says much when he's alone, off-camera, because think about his commentary on-camera. Or rather, his amount of commentary, and what prompts him to speak. If he's not ranting, feeling himself, or asserting himself, or telling a story that may or may not be true, partially or in entirety, he has nothing. It may be that in spite of stuff bothering him forever, he doesn't actually feel like expressing it unless someone is hearing it. There's a reason he used to share a lot more once, and it may be more connected to this than it seems at first glance.
Remember when he spilled water on himself? What if he had been alone, on his day off? Would he have had any words (uttered aloud, in the moment, not in a tweet later), whatsoever? What if he didn't? What he's silent about damn near everything until/unless he has an audience? What if it even extends to when he's stewing over something bothersome? He's very reactionary, on-camera or no, so it's quite possible that his mouth lives just as moment-to-moment as he does overall.
