Good news! After a long and arduous investigation, I was able to uncover a photo of the mattress in question. And here it is...
Armed with this vital visual evidence, we should now definitively be able to ascertain who truly was at fault here. The op has kindly marked off important areas of the bed to assist us in our investigation. The dark stain nearest to us is where the op generally sleeps. If you look towards the far right(where the blood stain is), that is where the laptop was hidden. The stain to the far left is where the teenage daughter claims to have been sitting. Finally, the burn mark that we see towards the back portion of the mattress is simply there from when the op fell asleep with a lit cigarette in her mouth(not to worry - she extinguished the flames with a can of beer). After seeing this detailed photograph, I am now confident in declaring that the damage was caused by a manufacturing defect in Dell computers.
And to the op - Teens are terrible creatures and cause much damage and heartache. In situations such as yours, you just have to tell yourself, "it was only a thing". Remember, a teen's sole purpose in life is to slowly strip away all layers of our sanity. Just hold on to this thought - one day she will have a teen.