Butterfly in the kitchen

This morning I discovered a butterfly in the kitchen.
First I thought he was outside, but looking better he sat inside fluttering on the glass window.
I wonder how he came there.
The window is always closed in the kitchen.
I quickly made a photo,

Then I opened the window,
He gratefully said: thank you very much...
waved a last good bye and then he fluttered/flying away into the blue.