RCCB (Residual Current Circuit Breaker)
aka RCD (Residual Current Device) and even RCB (Residual Current Breaker)
You are correct that sometimes with these RCCB/RCDs when they trip they may not look as if they have tripped, but once you have found the one that has tripped, press it all the way down to the OFF position before pushing back to the ON position, because that resets it properly.
As previously said the problem is finding out what was running at the time of the trip to be able to rectify the problem properly and as also said fridges/freezers are the worst culprits, obviously central heating thermostats should not be ruled out either because what can happen is that the contacts open or close slowly thus creating a spark across the contacts which then has a habit of tripping the RCCB/RCD.
Unfortunately, anything that turns off or on can be the culprit.
