I've been checking out my monitor here to see how the vents work. It has vents on the sides, under the rear part, and on top. It appears that the lower vents pull in the cool air, because the hot air is mostly venting out the top. Thus, it would definitely cause a big buildup of heat to block the top vents with a pillow on this monitor, and I figure yours was probably similar. It's an odd thing for housebreakers to do, and a very nasty one.