It could well have been the garage that didn't close it properly however I'm pretty sure if it wasn't shut properly then the alarm would have gone off and you'd have had the door open light on on the dash.

There is no way the garage would be found responsible for it though as like you say, there was plenty of time in between for you to open and close the bonnet. If you drove away from the garage and it happened 100 yards down the road it could be a different matter!