nyc cop robert hugel discovered his parents jerry , 83 and marianne hugel , 80 , dead in their queens home with ` carbon monoxide poisoning '
the elderly couple was found dead along with neighbor gloria greco , 70 , and friend walter vonthadden , 76 .
it appeared the car was accidentally left running in the garage , but it was unclear for how long , investigators said .
neighbors say the couple were happily married for 60 years .