Chances are you have nothing to worry about. The electrode most likely went out the exhaust valve and is now stuck in the catalytic converter (something this small will not cause an issue) or it passed entirely out of the exhaust system. If the car is running fine, not overheating, does not feel or sound like it is missing then I wouldn't worry about it. I don't think the part is stuck between a piston and cylinder wall. The normal piston to cylinder wall clearance is from .003 to .008 of an inch (it can be less or more depending on engine, components, intended use etc). The piece that broke off would have to be very small and land perfectly for it to be wedged in this small gap. If you want, you could have a mechanic do a compression check on each cylinder. This will show possible bent valves or other possible problems. Some mechanics also have a diagnostic scope or camera that can be put into each cylinder via the spark plug hole. With this tool you can sometimes see physical damage inside the cylinder....