Brake Master Cylinder Reservoir Levels.

I have just fitted new brake pads all round. 10 km. drive without using the brakes and stopped to check for dragging to find all four at about body temp. Gentle brake application on way back then allowed them to cool for the rest of the trip home. Checked for temp again and at about body temp so definitely not dragging. Great.
Fluid level in reservoir about 4mm below full mark as normal but I noticed a small amount of fluid had been expelled from the vent onto the top of the cap. Not enough to run down the side but a bit unusual as I've never seen this before.
I then removed the rubber cap to clean it and check the vent. The level was now UP by about 6 mm.
Has anyone had any experience with something like this ? What can be happening to cause this ?