We lost Jack the same way. It was likely a cardiac related event, possibly an embolism. We were aware that his heart was enlarged and our vet who took care of our Charlie told us in no uncertain terms his time was short before this. I still remember the day he told us, on a Saturday night after calling his cell (yes, Charlie brought us that close) on a picnic table outside his office. He was trying to break it easy to us that there was not a lot more we could do and just to give him the best life possible. We ended up losing him about two months later spending the summer at our beach house and he was happy minutes before he left and then he passed in my arms. I will never forget watching him and wanting him to come back.
I am sorry you lost Leo. It is very hard no matter how we lose our pups.