If you notice your body leaning forward even in a deep meditation you can make the adjustment to get your back correct again.
The experience of no thought are a stage of the meditation for those who have practiced over longer times.
Personally I have experienced this not only in sitting meditation but also in daily life because of the mindfulness of what is happening around me or when i doing a task. Noticing/being aware of the movement you do to the point that you do not have a past or a future you thinking of, only the moment you experience right now.
Thoughts are no longer important except if asked a direct question from other people around 
Once i adjusted my body a little upright, the tranquility sensation will be gone, and when i go into the thoughtless state again, the body fall forward again, so at this point i just ignore it and let it be.
I have a lot of questions regarding mindfulness in daily life activities, is inspiring to know that calmness can be achieve during daily activities. Would be grateful if you can share some experiences you had.
In daily activites,
1. When you are walking from point A to point B, lets say at work, do you watch(mindful of) your mind or watch (mindful of)your walking? If you watch your walking, sometime thoughts arises, how do you deal with that.
2. In a work place, sometime one will have a lot of movement like picking up the phone to speak and pick up a pen to take note, where to set the mindfulness?
3. When one is writing a email, one has to think of what to write, how can one be mindfulness while trying to "think" of what to write?.
4. If one superior is reprimanding one, if we set the mindfulness at the ear and acknowledge " sound", we will not be able to process the message the superior trying to bring across, then we wouldnt understand what needed to be done or correct? Or at this point we should be only paying attention? But if we pay attention to, we will receive the harsh word that make us upset.
5. For your experiences, do you work/move in a slower than your normal speed?
Hope my questions are not too bothersome

. Thank you.