1998 Chevy Lumina 98 Lumina has no electrical power

Electrical problem
1998 Chevy Lumina 6 cyl Two Wheel Drive Automatic

My sister in law has a problem with her 98 Lumina. She said a few weeks ago her hazard lights came on in the night and drained her battery. She has a new battery in the car now, (old one determined to have a dead cell) and after coming home from a 2 week trip the car is DEAD dead. She said they tested the battery and it has charge. They even tried to jump the car off of another vehicle and got nothing. She said they checked all the wires they could find to see if any were loose or detached, and she found none. I wish I could check it out myself, but I have to take her word that they did what she said.

Sooo, knowing what I know what could be the culprit? I would have thought jumping it would have created something (at least enough to turn on dome lighting) but she says no dice. Are the ground wires possibly undone, or not making good contact? She said the battery is new, and has no corrosion.

and what's with the hazards coming on by themselves? That's not something I would see being left on accidentally in the driveway, so what might cause that to happen?

Any help is much appreciated.

BH