Lost my little male schnauzer and now the house is really noisy

We were going to take the schnauzers for a Sunday drive. They love to see and bark at the cows, horses, geese, deer, and other critters along the way. My wife wanted to pick up a few items at Walmart first and as the weather was cool 65 and heavily overcast we thought they would be alright. Rowdy was in the front seat and Shadow was in the rear and the windows were down a couple of inches. We were gone less than 20 minutes. When we returned to the car Shadow was bouncing in the rear seat and Rowdy appeared to be asleep on the front seat, We unlocked the door and my wife said to Rowdy, "We're back sleepy bones". But Rowdy did not react. Touching him gained no response and that sickening feeling overcame both of us. He was dead. No signs of trama, nothing to indicate how it happened. We are both physically ill over this. He was only 12. Several years ago the Vet said he had a hearth murmur and we best keep an eye on it. The Vet checked him at every visit. This is the only thing we can fatham that might have been a cause. Perhaps someone got too close to the car and he started barking and they thought it funny so they egged him on.....to much. We really do not know but a dog you've had 12 years can take a tremendous amount of your heart when they leave you. Even though we still have Shadow this big old house feels empty and incomplete. And As I was saying the house has been even noisier since we buried Rowdy on Monday morning. A lot of it is probably me; but I hear rustling noises in the corner, I have detected tapping like a dog coming (the other dog was asleep) plus other nondiscript noises from all around the downstairs. I'm writing this primarily to get it off my chest. I hurt so bad I'd like to just die myself. But I have others depending on me. Perhaps God and Time will heal me.