I've owned both a Microsoft Surface laptop and currently own an XPS 15. Both of them I have had the same thing happen where they are in "sleep" mode and have woken up. I'll have them in a bag and open them up and it's really cooking inside the laptop sleeve and the laptop and bag are quite warm. Honestly the amount of heat there I wouldn't be surprised if there have been fires from this kind of stuff.

Basically if I know I won't be using my laptop for a bit and am putting it in a bag I always make sure to turn it off now.