Personally, I don't trust panes of glass or rental doors (where anyone could have copied keys) to keep my belongings safe. I have a small backpack that always contains my laptops and cameras, and it comes everywhere with me when I am traveling.

I don't leave it at the hotel, I don't leave it in the airbnb, and I don't leave it in the car. (Remember, the trunk of the car is one smashed window and four seconds away from being opened with the interior trunk release button.)

Then again, I also don't go rock climbing, so obviously my approach wouldn't work for everyone.

I grew up in a place where anything left anywhere in a car (trunk, passenger compartment, back seat, glove box, whatever), locked or unlocked, was fair game in 60 seconds. People who leave thousands of dollars of valuables in places without physical security other than panes of glass baffle me, but I also recognize that my experiences are unique in much of the developed world.

OP, don't beat yourself up over the unlocked window; even if it had been locked, the sliding glass door in the back was a one second barrier.