You're on the right track with how to find the answer. What you need to do is use your powers on the objects you have that would have been there at the time of Heather Anne's death.
Also, in case there's confusion, by "don't leave the testing area", it means don't leave the room. You can leave the screen close-up of the tablet without penalty. (We realized after release the wording for this is a bit confusing.)