How do you know your radiator coolant is leaking?

My Mitsubishi Lancer is almost 10 years old now. It has not given me any major headache or repair. The only time I have to replace is the timing belt. There are no issues with the suspension, brake, engine, gear-box, etc. It has an issue though that the horn will sound even without you activating it. That is a pain while imaging when at night, the horn will sound by itself and wake up the neighbourhood, or even worst, the police will knock on your door to ask you to deactivate the horn.

It is time for me to change my car. I still have about 6 months before I need to scrap my car. I will take back about $7K at the end of its tour cycle. It has served me well for the past 9.5 years and I have not regretted buying it.

During my last servicing at the garage, I was told that my radiator coolant was leaking. I didn't believe it as I did not have any issue with it. There are green stains on the radiator and based on the servicing agent's experience, the radiator is leaking. It will cost me $900 to change the radiator. The agent advised that since the car will be scrapped early next year, it is not worth it to replace a new radiator. He advised me to top up the radiator with water every morning before I start the engine. It is very troublesome and I tried to change the water level. There was no change in the water level for a week. I guessed the agent was trying to pull a fast one on me.

It is now almost 5 months since the last servicing. The car is due for servicing next month. I was stopping for a while to wait for someone and I noticed that the air-con was not cold. It started to become warm in a few seconds. I was thinking that the air-con was going to spoilt soon and did not take note of the car engine temperature. After my car moved off, the air-con became cold again so again, this did not cause an alarm to my mind.

Then, I was at a traffic red light and I stopped my car. Immediately the air-con turned to warm again, I looked at the car engine temperature. Normally, the indicator is at the middle of the range. Suddenly, the indicator went to the hot zone quickly. It was hovering between the three quarter mark near the H. I was panicking and thought that the radiator would shutdown and whole engine would stop. Luckily, the lights turned green and I managed to reach home safely. I wasn't sure whether it was the radiator problem but from the surface, it seemed like one. I couldn't open the radiator cap as it was still very hot and remembered not to open the radiator cap when it was still hot. It is better to let it cool down overnight and open the cap in the morning.

The next morning, I opened the radiator cap. True enough, the radiator was almost empty. I poured in about 600ml of distilled water. I was thinking whether I should buy some coolant water to top up instead. Any one has any suggestion? Thanks.

I went to the petrol kiosk and bought one liter of radiator coolant. I hope this will temporarily keep my engine cool and prolong its life till next year March. I was choosing between a Japanese brand and this local brand. Well, as long as it does not cause any problem to the radiator, it is ok. I will let post here whether I will encounter problem in the near future. 

Rad Kool Prediluted Longlife Coolant