I am aware of this type of hunting. Thermals still apply. In the evening you scent is getting pulled down into those low spots.
thermals are practically insignificant when there aren't any real changes in elevation, and don't really occur in flat country when the wind is blowing at all.
wind directions and air currents, on the other hand, are omnipresent everywhere. It never ceases to amaze me how little deer hunters know about the what the air is actually doing where they hunt. I don't leave the truck without my little container of milkweed fuzz. I pass lots of time in treestands studying what the air is actually doing in relation to what the actual wind direction is. air does weird stuff when it is blowing hard or barely at all, when you are hunting edges and inside/outside corners. knowing these relationships and understanding how to apply them to your benefit when tryin to ambush deer will increase your deer sightings by allowing you to get into better position to intercept them without getting smelled.
when you know deer are there and coming near your stand or blind but you never see them while hunting, it's more than likely because they are smelling you somehow before you can see them. all of this, provided you are only hunting an area when the wind direction is correct, you can get there undetected, sit still, yadda, yadda, yadda. late season is tough, but not impossible. granted, with time running out, you have to roll the dice a little, but your odd are always better when you understand the wind on your hunting ground.