Re: "check coolant level" warning - i need 1/2 gallon ???
OP - if there was dried coolant crystals on top of the header tank it could have been that the cap wasn’t on tight? Or the pressure valve in the cap is failing and letting the system boil out through pressure release hole. It could also be that at a certain pressure a crack is opening up somewhere. Did your expansion tanks actually blow due to excess pressure? Or did they leak, if the system is getting over pressurized and breaking other components I would get an exhaust gas test done on the coolant to be on the safe side.
Gaz - if there is no immediate tell tail, you could find that there is a leak in the radiator that only opens up under high temp/pressure and is then being sucked in by the viscous fan, hits the engine and evaporates so you never see the deposits. Have a look at the engine fan - does it have a clean patch on the blades where hot water may have been hitting it? If that's fine then maybe if all the other hoses and connections are good it's a heater union pipe (between the engine and the bulkhead) best place to verify this is from underneath. I had a small hole in a hose that would only open up when revved to over 2k, so it was fine when on the drive, but leaked when driving. Another point of failure that you may not notice immediately is the water pump, when the bearing starts to wear the shaft wobbles, this will then destroy the watertight seal and coolant will seep past the shaft and will boil off very quickly, again a difficult one to see!
To both - with the engine fully up to temp and left running pop the bonnet and squeeze the top hose. It should be firm, almost hard but should have a little give (remember the coolant temp is over 100°C and desperately wants to boil so its going to be hard) If your hose is squishy then you possibly have a leak in the system that will cause two problems 1. you will loose coolant 2. the coolant in the system will no longer be pressurized and when it gets to 100°C it will boil which will result in air bubbles throughout the coolant which doubles the coolant volume until it has to find a way out very quickly = burst hoses cracked header tanks.
The next thing to try if the hose is firm is to turn the engine off and try again in 5 minutes, the engine will still be hot and the coolant system should still be pressurized, if there is a small leak - the hose will be completely squidgy by now.