If a caregiver dies suddenly, as from a heart attack, the person in their care may be too infirm (or medicated) to assist them or even discover them. But that doesn’t explain one dead dog, one not.
Intruders can’t be ruled out, but this just doesn’t sound like a home invasion.
A lot depends on the pill contents.
If CO is ruled out then maybe a possible dementia related murder/suicide ?
“But that doesn’t explain one dead dog”
Dog ate spilled pills?