It depends on which section of the Satipattana Sutta you are referring to, and I would have to read the full section to give a more specific answer. But, in general to be able to be aware of the body and the body movements, the body sense doors, the hearing, the sight, the smell, the touch, the taste - this is all being aware, being mindful of the body. So it's perhaps applicable to say that when we're watching the body within the body, that in this practice we're just watching the physical actions going on.
We're just staying with the body awareness. So if you start reflecting about something else, then you're out of the body and you're into reflection, then just note to yourself, "Time to come back to the body." Just like when you're grabbing the door to open it and you start dreaming of coming back to Wat Kow Tahm to do another retreat. You're out of the body, you're not being mindful of your body, you're into your thoughts.
In a similar way: mind within the mind – when you're reflecting just stay with reflection. This is why whenever you're doing reflection meditation and a meditation pain arises in your body, in general, we say to change the posture - have it go away. For a lot of people with more experience they may get distracted a little, but they can just leave it there and they don't worry and they know how to handle it without looking at it, and they go back to their reflection. So when you're doing a reflection then the practice is to try to stay just with the reflection. That would be more mind in the mind. When you're doing a body action, grabbing the door, that's more body in the body.