first of all get the car coolant idea out of your head, coolant is actually a worse conductor of heat than plain water, the only reason it is used in cars is to lower the freezing point of the water and to raise the boiling point and to lubricate the water pump and seals, it has no use in a grow room, if you were to run pure anti freeze in your car it would overheat because it is a poor heat transfer substance, second you need to think of how a thermal immage camera works, it detects heat on the surface of things, if you are growing in a closet that is in a room the camera will not see the grow, now if your lights were burning in a room that shared a wall with the outside then the camera would see it, but in reality a 400 in a closet is not going to be a significant ammount of heat to draw attention, think if you were sitting there with a small heater going, that would put down 4 times if not more the heat footprint of the light, but the idea of bubbling it through water is good, then you have to figure a way to cool the water, otherwise pretty soon you have a big tub of hot water, so yeah if you put this together leave the car coolant in the car