Pills scattered on a countertop where Arakawa was found. Hackman found in the mudroom, possibly having fallen. One dog was dead, two others were alive.
Speculation:
Hackman falls, dies. Arakawa panics (I don't blame her), the stress causes some kind of cardiac failure. She goes for aspirin or something, fumbles it (or whatever pill bottle she pulled out), dies. One dog eats some of the fallen pills, also dies.
That kind of supports my speculation. Dog loves her, she's scrambling, dog comes. Ends up eating something it shouldn't have, feels ill, curls up in the closet. Dogs feel safer in enclosed spaces, "dens."
Idk, gas leak seems more plausible (even if it dissipated before tests happened). Maybe the wire felt lightheaded, went for pills, but didn’t have time.