Macbook Pro Didn't Sleep

I've just noticed that my Macbook Pro Apple light on the top cover of the laptop was on, and indeed despite the cover being closed the computer was not in sleep mode. It was warm, but not piping hot. I'm not sure what's going on but of course it means that I need to watch this machine a little more closely from now on. I'm presently doing an upgrade.

I should probably note here that the machine is still running Mint Linux which I installed a few weeks ago and despite this blip I'm almost certainly going to continue running this OS on what is now an aging laptop.