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I am so sorry that you have lost your pet. It is bad enough to go through this at all, but for a sudden death, without any warning signs is unthinkable. Unfortunately, you will probably never know for sure, but any of the things you mentioned already would not make me concerned at all. (vaccines, allergies, diet)
Dog's do not really have true "heart attacks" in the sense of the word as we humans do,
caused by atherosclerosis and a blockage in the blood supply to the heart. I think your vet may have been describing it in a way that you would relate to as it is possible his heart was involved. He could have been born with a problem called aortic stenosis (narrowing of his aorta) and been asymptomatic of all of these years. This is a problem that can be seen in retrievers. Sometimes a mild murmur is heard, but it could be easy to miss. This condition causes the left side of the heart to thicken and when it becomes so thick that it cannot function effectively anymore, they can go into cardiac arrest. This is similar to a young athletes that collapses and dies suddenly while exercising. I do not know if this is what occurred, but it is a possible cause of a sudden death with no symptoms leading up to it.
Another possibility could have been an abdominal tumor that may have been present and ruptured. If it was involving an organ like spleen or liver, he could bleed internally very quickly and collapse.
Toxins are also possible, but not many work that quick and as you mentioned he was in your yard, probably not very likely.
I know it is not easy to look back and think about what you might have missed as far as clues that you could have taken action on. It is very possible that there were none and you could not have done anything to prevent this. Please accept my condolences. Seven and a half years is not long enough but I'm sure he knew he was loved in his time with you. Please let me know if I can answer anything else for you.