Trash can woes

I had the exact same problem as you with Sims knocking over trash cans. It was happening way too much and on all lots I visited. I ended up finding a mod that prevents it if you use them. It's Bienchen's "noautonomouskicktrashcan" under the Autonomy Mods section and it works great. Other than using said mod, I haven't found a way to prevent it occuring so much. Hope this helps you for that problem. As for missing trash cans, are you sure Sims aren't swiping them with the Sharing is Caring N.A.P. from Eco? Other than that, I haven't heard or seen of trash cans just disappearing.