shifter

The volume of coolant will expand as it gets hotter (the reason you have an expansion bottle). If the volume in the expansion tank is too high it will force coolant out of the breather (it basicaly finds it own level).
It's nothing to worry about, just keep an eye on the level.
Don't forget as the weather gets hotter so will the bike. The coolant temp will rise quicker from 25c to 100c than it will from 2c to a 100c. For this reason it's probably worth checking the coolant level when the bike is up to temp.
Don't forget it's a pressurised system though so be carefall if removing lids.