I have a 20 amp circuit breaker feeding a GFCI outlet that continues on around the dining room connecting 5 other 15 amp receptacles with 12/2 wire. Each of these 15 amp receptacle boxes has (2) 12/2 wires with the receptacles daisy chained. I would like to figure out where the circuit continues on to for my own knowledge because I expected one to have only a single 12/2 wire just like every other room. It's mostly for my own curiosity and I want to make sure its not buried behind a wall or in the ceiling. From what I can tell every other fixture and appliance is accounted for. What is the best way to do this?
Nowhere is it written that electrical circuits must serve one room. In fact I consider it to be sloppy installation, as it wastes wire going down each shared wall twice, and limits your access to circuits (can only get to 1 circuit, that's how you get extension cords pushed through the drywall).
However, if the dining room is distinctively a dining room, the Electrical Code says branch circuits in kitchen, dining, pantry, larder, breakfast nook and the like must all be on 20A branch circuits that serve only receptacles, and only in those rooms. Code requires at least two branch circuits to cover those receptacles, and the vast majority of installers do the bare minimum. So expect the two kitchen SABC's to be busy beavers...
Your best bet is to turn the circuit breaker off and leave it off. It will soon become apparent which other outlets are on that circuit.
If the last outlet has two cables instead of one, like you expect, then the power might go to a ceiling or wall light.
It is possible that half of that outlet is switched, but should see signs of pigtail/wirenuts in the box.
Another possibly is that one outlet is split and is powered by two breakers, one sign is if the tabs joining both half's(top and bottom) are broken/missing.