I've seen my snakes get creases in their skin at times, but never such a long line like that! The "leaning on a counter" idea doesn't seem to make sense with such a long line down the middle of the belly either.

In the first pic, the belly looks reddish. Is the snake going into shed? Or is there possibly something else going on?

Is her body weight good? I've seen some pics of underweight snakes that have concave bellies. Perhaps if her belly was sucked in like that, and she laid in just the right position, it could cause a crease like that?