2003 Renault Scenic constantly filling coolant water bottle

Engine Cooling problem
2003 Renault Scenic Front Wheel Drive Manual 67.500 miles

I have been constantly having to refill my coolant device over the past couple of months, I do go longer distances from time to time but last week during the bad wheather a warning sign appeared and asked me to turn my engine off at the risk off engine failure as there seemed to be cold air coming from the heating system.
When I checked the coolant system there was no water in it and this seemed to be happening on a regular basis.
I handed it to my local garage who checked for leeks but none could be found which caused confusion, they also checked my head gasket and there seemed no apparent leakage on the gasket,
So the garage bled my car and asked me to try that and see what the outcome was, after driving my car for a week I checked the coolant and have noticed that the water had gone down but not to the extent of water I was going through before.
Should I hand it back to the garage or have you any thoughts on the problem.
Sorry about not being able to give the engine size as I am on night shift and I don, t want to creep around and waken the sleep in.
You could always e-mail me and I could check the manual and get back to you.

Billy