Losing a pet is a traumatic experience - you're essentially losing a family member. Even the stinkiest, most difficult-to-live with, or annoying of pets, when they're gone, leave a hole in your heart.
So much so that it's difficult to think straight when you see that they've crossed over to the other side and entered pet heaven.
But pets are animals, with biological structures that are different from our own, and there are many instances where we think our pets are dead, but they really aren't. Heck, we even make that same mistake with human beings every now and again.