I read somewhere that there is a screen in the fill neck so it's not possible to siphon gas. The auto parts store (Advance Auto) I went to before I found this out didn't list one. Toyota doesn't sell one.
There is a screen in them and that will prevent someone from sticking a hose in it. If they really want the gas they will just punch a hole in your tank and drain it. A locking gas cap can prevent punks or angry ex's from putting damaging materials in the tank. Peace of mind if it matters.
This is the one I installed and haven't had any issues with it throwing codes.