In my own experience of tripping RCDs, while there may be other causes, first and foremost be suspicious of any appliance that involves water. Specifically, in the 20 odd years these things have been around, I have tracked seemingly random faults as follows...
1). The 2-way valve in the central heating system. Clue was the trips sometimes followed after the room thermostat triggered. Easy to demonstrate once sussed, but seemed totally random until I fugured it out.
2). A cordless plastic electric kettle than had sprung a leak, allowing water to drip onto the base unit. Tripped ten minutes or so after boiling the kettle.
3). A dishwasher which was leaking water from an intrnal hose, dripping onto a circuit board. Clue was that it tripped about half hour into the dishwasher cycle, but took me an embarassingly long time to figure.
I hope maybe that provides some inspiration?