My Dell 9710 did that very thing today. Must have put it into standby instead of shutdown/laptop didn't shutdown all the way before I closed it. Threw it in my bag to go workout. Come back to my locker about 45 mins later and the fans are at full tilt 100% and the laptop was literally so hot I couldn't pick it up bare handed. Had to use my shirt just to open it up and power it down.Yeah but with connected standby Dell have screwed up sleep mode so shutting the lid no longer turns off the fans (S3 mode) meaning you have to shut it down fully or it will cook itself in your bag.
All new Dells are useless as laptops in that regard