If it is on concrete and NOTHING and Nobody is or will anywhere near be it, it should be okay. Then again, I'm a firm believer that if it can go wrong, it will. I will leave my Stumps unattended because it is all enclosed and it doesn't get hot on the outside.
I tell my customers that "it's almost set it and forget it". Leave and go to work for (I'm assuming) 8+ hours, not me cause I know my BDS's (Beautiful Drum Smokers) better than that. Take a nap or leave them for a few hours ... yes not a problem.
That's what makes cooking on a UDS, WSM, or even an ECB so worthwhile, once you get the feel of your smoker, you can actually sleep! I've even left my ECB all alone for 6 hours +, and it was still cruising at 240! It's about the cooker's technique, more than the actual piece of hardware used to cook. The extra sleep is a big bonus.