IIRC, the knife was a table knife, so that's understandable.
Maybe my household is weird, but when my now 28 y/o daughter was 16-22 and living at home, she would take food to work and my table flatware to eat them with - used to drive me crazy, because we had plastic flatware. Every so often I'd have to send her out to her car to bring in the cups, flatware, etc. when enough were missing that I noticed it! When she brought them in, I washed them!
What I do find a little odd is why Cindy was cleaning out the car. With everything else in daily life going on, even before she knew Caylee was missing, that would hardly be my first priority. Perhaps she's just more fastidious than I!