Okay. Like the title says, I'm not sure whether this is a theory or a headcanon but it is this: Janus either used to be a 'Light Side' or hung around with the 'Light Sides' a lot before Thomas met him. Just think about it a little.

Janus always knows where the Sides he's impersonating stand/where they are Summoned from and he also gets every little detail in their personality and appearence down almost perfectly. It takes the minimum of a year (for me anyway, it probably differs for different people) to start to recongnise and memorize different peoples personality and emotional patterns. And, this is just a guess, I don't think Janus' clothes can mimic other Side's clothes so he'd probably have to sneak into their rooms to get clothing. Like when he popped up as Logan but was wearing the wrong tie. If the Sides didn't trust him at least a little bit, I feel like their rooms wouldn't allow him entrance (I'll make a whole other theory on this later). Also, Thomas didn't know that Janus existed until 'Can Lying Be Good?' but everyone was yelling and accusing and hissing (in Virgil's case) at Janus when he'd revealed that he was impersonating Patton. That's basically it...

Please tell me what you think of this 'theory' in the comments.
Bye,
Queenie