17 plate Leon small coolant loss

[JRdrums]

I bought my car in may and have covered 20k miles in that time. When I purchased the car the coolant level when cold was at the crease in the tank. It’s now dropped gradually since then to the top of the hatched area which is a couple of centimetres lower. I know this is not a serious leak but is a small amount of coolant loss normal? Or do I need to get the dealer on the case before I’m out of warranty?