Mysterious coolant leak
I am currently dealing with a 2000 ranger 4x4 3.0l v6 its a relatives truck. HE told me that when he stopped at the gas station one day ( this is back during winter) that he noticed a pretty large amount of coolant leaking out of the front of the engine on the passenger side. He quickly added water to the radiator and drove home. not having the money to have it looked at at that time he just drove back and forth to the sotre and added water as needed, truck never over heated. So now for my issue, I cant find a leak, drove the truck for a few miles got it hot etc. no leak, pressure tested no leaks either. the coolant in the resivoir looks brownsh, maybe oil contaminated, there is no coolant in the oil. my qiestion is does anybody have an idea as to whats going on or where to start?, this is mind boggling.