2015 Forester overheat radiator

BACKGROUND: Cool day, 65 degrees. Driving on Interstate at 70 mph. The engine overheat light came on.

Lifting the hood, we see antifreeze moisture was blown around the engine compartment, but the radiator was not boiling over. The coolant resovoir had the right amount, neither high not low. Took the radiator cap off and, of course, didn't see any liquid. The upper and lower hoses seemed in good shape tightly clamped. Got towed off the tollway.

I added 1/2 gallon water directly into the radiator with engine running, to see where it might be leaking. Engine ran for 20 minutes--no drips or leaks, no overheat, but the cooling fans did not run either. Turned on the AC and the fans started up, so apparently they work.

Drove home, two hour drive. No problems, though it did take a few cups of water when trip was done.

QUESTION: What might have gone wrong to cause coolant lose and overheat? What do we need to do to avoid future problems? Thanks for replies.