As the title suggest that is what is happening to my 07 F150 with 120k miles. I have topped off the reservoir several times and have looked at the hoses, radiator, water pump, etc and have never found the slightest leak. No smell of coolant inside the truck or outside. Truck runs good and has never been run hot. Has anyone used the coolant UV additive with any success? Any feedback on that. Is a cracked intake manifold a possibility on these models? I know Ford had an issue on earlier models.

 

I'd run a hydrocarbon test on the coolant, you may have a leaky head gasket.

 

Well after several weeks of adding coolant. I think I just found the problem. The truck is normally parked in a gravel driveway. Just now I drove it onto a concrete apron that slopes upwards to my garage. Immediately there was a water puddle right under the bell housing. I crawled under to confirm and it was not ac or exhaust condensate but green coolant. Makes me think it is the intake manifold more now and it is collecting in the valley of the engine and when I drove it nose up into the slope it ran out the back. There are traces of when water has run for a little while. I am thinking this is good bad news as diagnosis is half the problem and it may not be a head gasket.

 

It might be the water coolant tube that runs underneath the Intake manifold in the valley from the back side of the Water Pump to the rear of the Intake manifold and connects to one of the Heater hoses. The O-rings can go bad.

 

I had a 07 SC with 4.6l I ran to 217k before selling to get my15.
the 2007 4.6l has a known issue with the plastic part of the intake manifold where the thermostat is located. It cracks and allows coolant to leak.
Mine failed at about 100k . Installed a new improved part and problem solved.
this was several years ago so there should be plenty of info about it on the net.
good luck.