rxed

Hi
This is rxed's Mum - he is at work. He parked car in car park - 20 minutes later there was coolant on ground coming from passenger side - checked car over can't find any leaks - coolant level not dropped by much. Question is - when you look into engine bay on the left hand side of engine bay behind headlight is white filler neck going to a tank which is below headlight behind oil cooler. What goes in this tank? He is still at work trying to find leak so answers asap please.

This is a jdm car.

Thanks and look forward to hearing from you