b-max coolant reservoir level keeps changing.

My B-Max 1 litre Eco boost is 7 years old (approx 68,000 mls on the clock)

This has started to occur in the last few months of ownership.

After driving the car and allowing the engine to reach NORMAL operating temperature I park the car for some time (allowing engine and coolant to cool). I check and monitor the coolant reservoir level.  I note the level in the reservoir (A). I drive the car again and allow the engine temperature to reach NORMAL operating temperature. I park the car again for sufficient time to allow the engine and coolant to cool. I find the reservoir level (after cooling) is approx 2cms higher than the previous check at (A). 

Sometimes the level drops between two inspection periods by approx 2cms. It doesn't occur every journey. Sometimes I will drive the vehicle for a week or two and the level will remain predictable and stay at the same position, when engine and coolant are fully cooled. 

I realise levels will rise after the fluid has heated (eg expanded after a journey) but I would expect the level should to drop to it's previous level after the fluid has fully cooled but it doesn't always do that, it stays high or sometimes lower than the previous check.

 I have owned the car for 4 years and have never noticed this occurring previously.  I am keen at keeping an eye on all the mechanics of the car and have never noticed this on other cars I have owned. I cannot find any coolant system leaks and it doesn't appear that a head gasket is leaking as the coolant system doesn't appear to get pressurised in any way. No contamination of the cooling fluid either.  Or maybe this is normal for a B-Max?

One puzzled owner would appreciate comments.