I am saddened by the loss of my good and faithful Dog "Bubby" .... I came home from work to find out that he had passed away... That was sooo unexpected ! He was actively healthy, ate well,... and now he's gone !
Heart failure ??? Spent 11 yrs with him..... I miss him already !!!

I am so sorry for your loss. You could have your vet do an autopsy on him for peace of mind.
My sister's dog passed away last Friday. Same thing, she came home and Squeakers was in her crate no longer with us. When she told my nieces, the four year old had some great words of wisdom. She said, "I don't miss Squeakers because I know she is up in heaven with God and Jesus." Now, those are some powerful words for a four year old.
Take care and I am sure that your loyal friend is having a grand 'ol time with Squeakers!