What should I do if I forget to close the hood when it rains lightly?

It doesn't matter.

As long as the ignition coil is not wet, the temperature in the engine compartment is very high and the water vapor will evaporate quickly.

If the vehicle encounters problems, please ask professionals to deal with them. Don't disassemble the parts of the vehicle without permission to reduce the damage to the vehicle. Even if there is damage, you can get compensation from the insurance company.