In 1966 my dad bought a new Polara, not quite as fancy as the Monaco. He ran that car for many years. Sold it cheap to my mother-in-law; she was just learning to drive. After she was done beating the hell out of it and on it's last legs they gave it to me. Got it running and driving well and used it as my daily driver/work car. Ran it for the next few years and then sold it. That car always ran great and was comfortable to drive. But I never found anything like that in the trunk...........................EVER!
I've got a suspicion that you Dad took it out and kept it for himself. You should have checked HIS trunk!