I did something similar when I was 10 or 11. We had bbq'd and I told my dad that I would clean up. I too thought the coals were out so I took the tin foil wrapped leftovers and threw them in the trash can, in our garage, next to my parent's new car. A little while later we were sitting on the porch talking to a neighbor, ironically about a string of arsons in the area, when the neighbor said, "I can smell smoke now". Yup, the trash in our garage was on fire. My dad was not too impressed with my clean up skills! The car, a new to them 1976 Buick Electra did not get damaged.