If you are really sure that when you checked the temp and it appears normal yet hot air blown out from aircond means that your radiator fan has failed. The radiator fan is also the a/c condensor fan if not mistaken. When that fan fails you will lose aircond cooling and engine cooling together. Correct me if im wrong.
Intermitten failure of cooling fan can cause random and hard to trace overheating. Other things related to fan are fan motor, temp sensor, ecu fan signal (if fan operates by ecu trigger), fan relay, fuse, wiring n contact points

Loss of coolant...
You mention radiator tank, not radiator itself. If you're talkin about the 2ndary coolant tank near the radiator loosing coolant after overheating, its normal...it was circulated into radiator and engine to accelerate heat transfer.

If you were refering to the radiator water level itself, i dont think you can see thru the bottom to know it is 'empty'. there may be alot of water below the level you can see with naked eye. Did you try to top up? How much did you need to top up? If it was really empty you will need 5L plus or so of water.

From experience it can be a few things, simplest things will be worn out radiator cap, hoses, drain plug, thermostat and it's housing area. If the fan failed and these parts are worn out, they may leak water.
More serious things, somewhere cracked on the metal parts of your radiator or engine which only opens enough to leak water when hot enough. Internal engine failure/damage like what others mention above.
It can also be leaking thru your water pump area which is usually covered, driven by timing belt/chain.

Others in thread hav mentioned almost all causes, best to find a better mech as this is basic stuffs. Btw, your mech any chance in damansara area? His style simillar to one mech i know many years ago, lost 10k+ to him the hard way.

This post has been edited by andrew9292: Jan 2 2014, 10:50 PM

Well sometimes cannot fully blame the mechanic. Some mechs will do full cooling system service eg.radiator service, fan motor, thermostat, hose and links, radiator cap replacement, temp sensor, water pump and top overhaul together. Some may even say engine overhaul only but charge u extremely high and do the full service quietly, as the other parts are relatively much cheaper should he use oem or non original parts.

Why? Cuz it completely solves the problem and should last half a decade at least if job done properly. Saves time and prevents customers from fucuking them upside down if cannot solve the problem/ taking too long, plus its a long lasting solution. Altho if you did specify u're on a budget or to troubleshoot root cause the correct thing to do is start from engine external, if failed then only engine internal. With proper troubleshooting and inspection methods of course.

Thats my only argument for the mechanic to be 'fair'.lol. but he still shouldnt jus jump to top overhaul and all that nonsense. Good luck!

This post has been edited by andrew9292: Jan 3 2014, 02:41 PM