Adele Ridless, 69, and her boyfriend Mort Victor, 79, were found dead in their Boca Raton home March 1, the South Florida Sun Sentinel reported.
The house was filled with dangerously high levels of carbon monoxide, the Palm Beach County Sheriff's Office said.
Investigators found the keyless Mercedes in the garage and the car was impounded to be analyzed.
The car was not running when authorities arrived, but investigators said it could have run out of gas.
Cars with keyless entry systems have been linked to carbon monoxide deaths and injuries in at least four states. When left on, the cars produce the invisible, odorless gas which can get trapped in the garage and then leak into the attached house.
Critics say the push-button ignition makes it too easy for drivers to leave their cars on and not all models have warning systems when drivers do forget.