So during EotE character creation, all new players receive 500 credits to work with (plus any additional credits they took for extra obligation) as per the core rulebook.

However, after doing some homework whilst preparing to GM a new campaign, I've seen this thing around (forums/reddit/notes etc) where players can roll a d100 for extra credits or "pocket money."

I actually like the sound of it - it gives the illusion that characters have a life of their own beyond the players and it suggests they've done things in between campaigns or before a new character meets up with the group. I just don't recall seeing this in the core rulebook and I'm wondering if I've missed a paragraph somewhere, or if it's not an official rule, I'm curious as to where the concept might have started.

It's a rule. I've seen it, I just don't remember where.

Also, important to note: the d100 extra cash is for after they've spent the starting cash. They can't spend it before play; they need to find a store in-game or wait for after the session, if that's how you like to play,

It's on page 98 of the CRB: "In addition, once the player has finished spending his starting credits, he rolls 1d100. His character gains this many additional credits as 'pocket money' at the beginning of the game. This additional money may not be spent during character creation."

Wow, I really did miss that. Thanks for the clarification!