What is the difference between 'Witnessing' and 'Mindfulness' from the context of meditation?

Since for my last question , I did not get a satisfactory answer I am reducing the question to its barebones. What is the difference between 'Witnessing' and 'Mindfulness' from the context of meditation? I mean when I am looking at the sunset without any thoughts in mind and feel a oneness, am I witnessing the sunset or I am being mindful of the eye-consciousness? In the Mahāsatipaṭṭhānasutta MN 10 the word mindfulness is used, can I replace it with the word, 'Witnessing' without changing the meaning?