Insurance & open Window

Depends on your policy terms & conditions. They should clarify what is and what is not covered.
 
More than likely based on ts and cs I have read lately. You are in fact meant to have windows locked and keys out of the locks in a lot of cases.
 
Hope you don't mind my asking but have you an alarm system and was it turned on? The reason I'm asking this is because Phonewatch didn't alarm two windows in my house because of their height and inaccessibility from the ground. A tall ladder would be needed to get in. I'd imagine the same would apply to a skylight.
 
Yup, house alarmed and motion sensor picked them up. They pulled the alarm noisebox off the wall and kept going.....