I usually find a secret map for Twikkii Island at the campground in Three Lakes. That's all I can really tell you, that's the only map I've ever found...
I don't think that the maps were ever hidden in a tour. For the secret lot, I have no idea how to use the maps. Try taking the map out of the sim's inventory and put it somewhere (only if they're at a vacation house). Sorry I'm not much help.