OH NO! Duck for cover! Okay, all humor aside, there is nothing to worry about. If you were filling in the reservoir, the foil is going to live there forever because it will never fit through the hose going to the radiator. If you were filling in the radiator, it won't fit through the cooling tubes to go any further. The only thing it could do even if it was able to get through the radiator is partially block the thermostat, and I doubt you would even notice that. If it did get as far as the water pump, the impeller would laugh at it and shred it up. There's already sand in most cooling systems that didn't get fully cleaned out during the casting process. That doesn't cause problems either.