Mysterious Intruder

My heart is still pounding! Just got in from shops to find a starling in my lounge! It kept flying against the windows to get out, then around the room again. Did a bit of starling wrangling (with a newspaper and waste bin) and managed to get it out of the window safely.
But how did it get there in the first place - all the windows were closed (and the cats didn't go out last night)!!!