I have a 99 F250 4x4 SD 320K that is loosing water. I could find no water leak; so, I took it to a Ford dealer and they have been unable to find a leak. It was pressure tested hot and cold.
I have added water to the resevior several time. I know there is a problem.
One doesn't go for months without putting water in to all the sudden adding water several times in a few days. When I first noticed it, the reservior bottle was half empty ( or half full if you want to be optimistic about it).
There is no water in the oil.
TruckoffFord

 

Keep an eye on your dipstick to make sure your oil doesn't start getting fuller. If I had to take a wild guess, I'd say your water pump may be on it's way out. You can develop a small leak at the water pump that mostly drips on the road while driving, but a hot engine helps evaporate the water so there's no puddles in the driveway.

You can try starting it up cold, revving the motor a few times, shutting it off before it's warm and then crawl under the truck or use an inspection mirror and check for any signs of water around the pump.

 

Thanks, I'll give it a try.
TruckoffFord

 

also look real close at the flange where your upper hose comes into the water pump that is where mine started leaking changed it for like $30 from napa