Engine coolant light comes on ?

Peugeot 207- year 2006
My engine coolant light has come on & message to STOP has been flashing and I don't know what to think or do. This occurred previously in Nov.

At that time I was able to park it at home and examine the following day. The fluid reservoir was just below the low mark so I topped up with some water & headed to work. 20 minutes into the journey the warning light came back on so I took it to my mechanic. They performed a dianostic test and ran the engine and drove the car - but the sensor light would NOT light up. No problem found.

I have just had the same problem occur tonight, almost 2 months after the first occurence, and I dont know what to do. Fluid levels appear normal.

Do you think it needs more antifreeze? If so, could I use a store brand instead of taking it to the dealers. I would think that I could but the dealers have a good way of making you think that going elsewhere would be worse for your car. Please can any one advise?