With state of decay, unless you actually complete that mission before you save and quit out, it wont be there when you get back. Quests are random, it may appear again later, but unless you do a mission in a play session, don't expect it there when you close the game and open it back up the next time.
I didn't close the game though. Just paused and turned the xbox off. It would usually auto resume. I found my car i think, looked similar and had petrol and tools in the boot as I left it but needed repairing. It was quite a distance from where I left i though. The quest just popped up again luckily.