1997 Ford F-150 leakage

My truck started leaking the other night out of the blue. No signs of a problem until I got up the next morning and saw goldish yellow stain on the driveway indicating something had been leaking. I looked under and no drips -- drove the car about 4 miles and when I got to my location and parked, saw a small puddle of same fluid. I parked there all day and that night, drove back same way. About 1/4 mile from ending location, I noticed the coolant light go on inside. Then suddenly the temp guage started going up into red. Stopped and let engine cool off for an hour and add water to coolant resevoir -- its was empty. Towed it into auto shop because a mechanic friend thought it was hose, but could not locate leak. When you shake the truck back and forth (LOL!) This fluid poured out onto driveway. The mechanic has been on it for two hours and can not find problem. Is taking "everything" out tomorrow to get to some "pipe" behind the engine that pumps water into the engine. Can anyone shed any light on this? Obviously, I will have them flush the radiator, but what does it sound like Im dealing with here? Other than the above sequence of events, there have been no other problems or indications of such. Thanks for your help.