[Steve Tinker]

You may have an air pocket in the cooling system - be very careful not to overheat the engine with this loss of coolant..... How does your oil level look - any indication of water in coolant (milk shake), indicating an internal coolant leak.

1) Try cleaning the inside of the tank using a rag dipped in vinegar - try a screwdriver if your fingers are short - just enough so you can see the level
.
2)Top the coolant up with distilled water.

3) Under the black plastic cover in your photo, you will find an expansion valve with s chrome D shaped ring - lift the ring up into the vertical position (that's open) and drive your car for 3 - 4 days to expel any trapped air. Check the coolant level daily until it stabilises.

4) Return D ring to its close position.

5) If the level still goes down, you have a leak - return it to your mechanic pronto.