Weird... I responded to this post this morning but it seems to have disappeared.
You're probably right about watching the smoke more closely, but I sort of figured that the whole point of having a pellet grill with a large hopper and fancy digital control was so that I could leave it unattended... While I do prefer charcoal, I can't have one on the deck of my wood-framed apartment building. The Traeger is safer than a gas grill in terms of a potential fire hazard.
I was fine all night and today, and have heard back from most of the people and no-one had any ill-effects - so it looks like I "got away with it". I'll definitely keep that 4hrs to 140degrees rule of thumb in the forefront of my mind, as well as not needing to probe for the first several hours and monitoring my smoker more closely. I'd still like to know why the fire went out, but after I restarted it it chugged along for ~8-9hrs with no issues, at which point I shut it down.