To do that properly you need to remove the neutral from the bar that goes with the mcb, turning off the mcb only removes the live, if its a fault on something you missed or a cable it will still trip, you must remove the live and the neutral when trying to locate or rule a circuit out.
Rcd just acts like a balance, its watching the electric and expects the same on live and neutral, you could have a situation when you get earth leakage that you didn't think possible ie tv with no earth but a fault that sends voltage via aerial cable to earth and rcd goes, I know, I've had that before.
Usually it's something obvs like outside light, fridge toaster, sometimes it's a mare, we had a fault on a ring main a mate of mine couldn't find it (proper lecy) and we had to split the circuit and loose a couple of sockets, I found it when I did our bedroom last year and the feckers before us buried a joint in a plaster wall.