1. Refill your expansion tank about 1/2 way and add coolant as necessary. When the engine is cool, also remove your radiator cap and check your coolant level. If low you'll want to refill the radiator as well. but read the below suggestion first.
2. There is no warning when the expansion tank is low/empty. Only if you overheat. As with checking your other fluids, this is something you should do when you buy fuel. Especially on an older vehicle.
3. You have a coolant leak somewhere and it is probably not your radiator, as it's drawing coolant from the expansion tank.
Depending on which engine you have there are more common leak areas. The 5.0L V8's tend to develop a leak where the timing cover gasket is very thin and coolant passes from the water pump to the engine block. If this is the source of your leak you can have the gasket replaced, but it is a pretty big job. Other typically coolant leaks are hoses and water pumps.
If you believe the leak is your timing cover gasket I suggest you try this first. Go to your local auto parts store and buy an 8 oz bottle of something called K-Seal. Except no substitutes. K-Seal typically costs around $15. When the engine is cool, remove the radiator cap, shake the K-Seal bottle very well and poor all of it in. Fill the radiator and expansion tank will coolant (expansion tank no more than 1/2 way), start the engine and let it idle for 30 mins.
I had a pretty good leak from my timing cover gasket, used K-Seal 3+ years ago and the leak stopped and has never returned. I've recommended K-Seal to others on the forum and they have also reported success with this product. It will not hurt anything, is considered a permanent repair and remains in the cooling system to prevent future leaks.
I've never been a big believer of "fixes in a bottle" but K-Seal really works and has saved me a lot time, money and aggravation.
